Internal Medicine Practice Questions — USMLE Step 2 CK — Page 89

Match each patient's clinical and laboratory presentation to the most likely underlying pathophysiologic diagnosis.

Click a left item, then click its matching right item

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Answer

Profile 1 matches Autoimmune adrenalitis; Profile 2 matches Exogenous glucocorticoid suppression; Profile 3 matches Aldosterone-producing adenoma; Profile 4 matches Cortisol-producing adrenal adenoma.

Each clinical scenario presents a distinct disruption of the adrenal axis. Primary adrenal insufficiency is characterized by failure of the entire adrenal cortex, resulting in low cortisol (raising ACTH) and low aldosterone (causing hyperkalemia and hyponatremia). Secondary adrenal insufficiency features low ACTH and low cortisol, but normal electrolytes because aldosterone is controlled by RAAS, not ACTH. Primary hyperaldosteronism autonomously secretes aldosterone, suppressing renin and causing hypokalemia. ACTH-independent Cushing syndrome autonomously secretes cortisol, suppressing ACTH.

Step-by-Step Solution

1

Evaluate the cortisol and ACTH levels for the first two profiles to differentiate primary from secondary adrenal insufficiency.

Profile 1 has high ACTH/low cortisol (primary), while Profile 2 has low ACTH/low cortisol (secondary).

The direction of ACTH relative to cortisol identifies the source of the defect (adrenal vs. pituitary/hypothalamic).

2

Analyze the electrolyte panels for the adrenal insufficiency profiles.

Profile 1 shows hyponatremia and hyperkalemia (aldosterone deficiency), confirming primary adrenal failure. Profile 2 shows normal electrolytes, indicating intact renin-angiotensin-aldosterone function.

Aldosterone secretion is primarily regulated by angiotensin II and potassium, not ACTH, so it is preserved in secondary adrenal insufficiency.

3

Assess the hypertensive profile (Profile 3) for the renin-aldosterone axis.

High aldosterone and low renin identify an autonomous adrenal source of aldosterone.

Autonomous aldosterone production suppresses the juxtaglomerular apparatus, dropping renin levels.

4

Assess the hypercortisolism profile (Profile 4) for ACTH dependency.

High cortisol with suppressed ACTH points directly to an adrenal tumor.

Autonomous adrenal cortisol production exerts negative feedback on the pituitary, suppressing ACTH.

Key Concept

Differentiating adrenal gland pathologies requires analyzing the feedback loops between the pituitary (ACTH), the adrenals (cortisol, aldosterone), and the kidneys (renin, electrolytes).

Question 1762Question →

A 42-year-old woman with acute myeloid leukemia presents to the emergency department with a fever. She received her last cycle of induction chemotherapy 12 days ago. She reports no cough, dyspnea, abdominal pain, or dysuria. Her temperature is 38.8°C (101.8°F), blood pressure is 118/76 mm Hg, pulse is 98/min, and respirations are 16/min. Physical examination reveals a right internal jugular totally implantable venous access port with no overlying erythema, induration, or tenderness. The lungs are clear to auscultation. There are no oral ulcers or skin rashes. Laboratory studies show:

Hemoglobin: 9.2 g/dL
Leukocyte count: 400/mm³
Absolute neutrophil count: 120/mm³
Platelet count: 45,000/mm³

Blood and urine cultures have been obtained. Which of the following is the most appropriate next step in pharmacotherapy?

Administer intravenous cefepime and vancomycin

Administer oral ciprofloxacin and amoxicillin-clavulanate

Administer intravenous cefepime

Administer intravenous ceftriaxone

Administer intravenous piperacillin-tazobactam and fluconazole

Show answer & explanation

Answer: Administer intravenous cefepime

Answer

Administer intravenous cefepime

The correct response is to administer intravenous cefepime. This patient has febrile neutropenia, an oncologic emergency requiring prompt administration of empiric antibiotics after obtaining blood cultures. Because she recently underwent induction chemotherapy for acute myeloid leukemia, her neutropenia is expected to be prolonged (>7 days) and profound, placing her in the high-risk category. High-risk patients must receive intravenous antipseudomonal monotherapy. Acceptable agents include cefepime, meropenem, or piperacillin-tazobactam. The patient lacks indications for the upfront addition of vancomycin.

Step-by-Step Solution

1

Calculate the absolute neutrophil count (ANC) and define the clinical syndrome.

The patient has a fever (38.8°C) and an ANC of 120/mm³.

Febrile neutropenia is defined as a single oral temperature of ≥ 38.3°C (101°F) or a sustained temperature of ≥ 38.0°C (100.4°F) for over 1 hour in a patient with an ANC < 500/mm³ (or anticipated to decrease to < 500/mm³).

2

Stratify the patient's risk to determine the route and setting of antibiotic administration.

The patient is high-risk due to profound (ANC < 100/mm³ or expected to be) and prolonged (> 7 days) neutropenia associated with acute myeloid leukemia induction therapy.

High-risk patients require admission (if not already hospitalized) and immediate intravenous broad-spectrum antibiotics.

3

Select the appropriate initial empiric antibiotic regimen.

Intravenous cefepime (an antipseudomonal beta-lactam) is selected.

Initial therapy must cover Gram-negative organisms, particularly Pseudomonas aeruginosa. Routine addition of Gram-positive coverage (vancomycin) is not indicated unless specific risk factors (e.g., hypotension, port-site infection, severe mucositis) are present.

Key Concept

High-risk febrile neutropenia requires immediate empiric intravenous therapy with an antipseudomonal beta-lactam, avoiding routine initial use of vancomycin or antifungals unless specific indications exist.

Question 1763Question →

A $20$ -year-old man with a history of type $1$ diabetes mellitus is brought to the emergency department due to nausea, vomiting, abdominal pain, and increasing lethargy over the past $24$ hours. He recently had a mild upper respiratory tract infection. Vital signs show a temperature of $37.8^\circ\text{C}$ ( $100^\circ\text{F}$ ), blood pressure of $90/60$ mm Hg, heart rate of $128$ /min, and respiratory rate of $28$ /min with deep, rapid breaths. Physical examination reveals dry mucous membranes and delayed capillary refill.

Initial laboratory results are as follows:

Laboratory Test	Result
Serum glucose	$580$ mg/dL
Serum sodium	$132$ mEq/L
Serum potassium	$3.1$ mEq/L
Serum bicarbonate	$12$ mEq/L
Arterial blood gas pH	$7.18$
Serum ketones	Positive

Arrange the following therapeutic interventions in the most appropriate chronological sequence for the management of this patient.

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Answer

The correct sequence is: rapid infusion of normal saline, intravenous potassium chloride supplementation, continuous intravenous regular insulin infusion, addition of dextrose to IV fluids, and transition to subcutaneous insulin.

The management of diabetic ketoacidosis (DKA) follows a strict algorithmic sequence. 1) Fluid resuscitation with isotonic saline is the most critical initial step to restore perfusion. 2) Because this patient's potassium is severely low (

< 3.3

mEq/L), potassium replacement must occur before insulin administration. 3) Once potassium is safe, continuous IV insulin is started to treat the acidosis. 4) Dextrose is added when glucose reaches

200-250

mg/dL to avoid hypoglycemia while insulin continues to clear ketones. 5) Finally, subcutaneous insulin is overlapped before IV insulin is discontinued once the anion gap is fully closed.

Step-by-Step Solution

1

Assess the patient's volume status and begin fluid resuscitation.

The patient is hypotensive (

90/60

mm Hg) and tachycardic, indicating severe volume depletion. Intravenous

0.9\%

sodium chloride is initiated first.

Restoring intravascular volume improves tissue perfusion and facilitates the renal clearance of glucose and ketones.

2

Evaluate the initial serum potassium level before initiating insulin therapy.

Serum potassium is

3.1

mEq/L. Potassium chloride is administered next, and insulin is temporarily withheld.

Insulin drives potassium into cells. Administering insulin when potassium is already

< 3.3

mEq/L can precipitate fatal cardiac arrhythmias and respiratory muscle weakness.

3

Initiate insulin therapy once potassium is in a safe range (

\geq 3.3

mEq/L).

Continuous intravenous regular insulin is started.

Insulin is required to suppress lipolysis, halt ketogenesis, and resolve the underlying ketoacidosis.

4

Monitor blood glucose and adjust intravenous fluids accordingly.

When glucose drops below

200-250

mg/dL,

5\%

dextrose is added to the maintenance fluids.

This prevents hypoglycemia while allowing the continuation of the insulin infusion, which is still necessary to completely close the anion gap.

5

Determine readiness to transition off intravenous insulin.

Once the anion gap closes, pH normalizes, and the patient can eat, subcutaneous insulin is administered.

Subcutaneous insulin must be administered

1

to

2

hours prior to stopping the IV infusion to prevent rebound ketoacidosis.

Key Concept

In the management of diabetic ketoacidosis (DKA), if the initial serum potassium is

< 3.3

mEq/L, insulin therapy must be delayed until potassium is aggressively replaced to prevent potentially fatal hypokalemia.

Estimated Time:1m 30s

Question 1764Question →

A 62-year-old woman comes to the clinic for a follow-up visit. Two months ago, she was prescribed hydrochlorothiazide for hypertension. She now complains of generalized weakness and occasional muscle cramps. Her blood pressure is 118/76 mm Hg, and her pulse is 78/min. Physical examination shows dry mucous membranes and no peripheral edema. Laboratory studies show:

Test	Result
Sodium	138 mEq/L
Potassium	2.9 mEq/L
Chloride	92 mEq/L
Bicarbonate	34 mEq/L
Arterial pH	7.52
$pCO_2$	48 mm Hg

Which of the following best describes this patient's acid-base status?

Primary metabolic alkalosis with appropriate respiratory compensation

Primary respiratory acidosis with appropriate metabolic compensation

Primary metabolic alkalosis with a superimposed respiratory acidosis

Primary respiratory alkalosis with appropriate metabolic compensation

Primary metabolic acidosis with a superimposed respiratory alkalosis

Show answer & explanation

Answer: Primary metabolic alkalosis with appropriate respiratory compensation

Answer

Primary metabolic alkalosis with appropriate respiratory compensation

This patient has a pH of 7.52, indicating alkalemia. The elevated serum bicarbonate (

34 \text{ mEq/L}

) identifies the primary disorder as metabolic alkalosis, likely secondary to hydrochlorothiazide use, which causes volume contraction and hypokalemia. The expected respiratory compensation is calculated using the formula: Expected

pCO_2 = 40 + 0.7 \times (\Delta HCO_3^-)

. Here,

40 + 0.7 \times (34 - 24) = 47 \pm 2 \text{ mm Hg}

. Since the patient's measured

pCO_2

of

48 \text{ mm Hg}

falls squarely within this expected range, she has a simple primary metabolic alkalosis with appropriate respiratory compensation.

Step-by-Step Solution

1

Analyze the pH to determine the primary disturbance.

The arterial pH is 7.52.

A pH greater than 7.44 indicates that the primary acid-base disorder is an alkalosis.

2

Identify the primary driver of the alkalemia.

The serum bicarbonate is elevated at 34 mEq/L, and the pCO2 is elevated at 48 mm Hg.

An elevated bicarbonate directly causes alkalemia, identifying the primary disorder as a metabolic alkalosis (often caused by thiazide-induced volume contraction). The elevated pCO2 is a compensatory response.

3

Calculate the expected respiratory compensation.

Expected

pCO_2 = 40 + [0.7 \times (34 - 24)] = 40 + 7 = 47 \pm 2

mm Hg.

Because the patient's measured pCO2 (48 mm Hg) falls within the expected range (45-49 mm Hg), the respiratory compensation is appropriate and there is no superimposed secondary disorder.

Key Concept

Thiazide diuretics can cause volume contraction and hypokalemia, leading to a primary metabolic alkalosis. The expected respiratory compensation for metabolic alkalosis is an increase in pCO2 by roughly 0.7 mm Hg for every 1 mEq/L rise in bicarbonate.

Question 1765Question →

A 65-year-old man is brought to the emergency department by his family due to a severe headache, fever, and progressive confusion over the past 12 hours. His temperature is 39.2°C (102.6°F), blood pressure is 110/70 mmHg, pulse is 112/min, and respiratory rate is 18/min. On examination, the patient is lethargic and oriented only to person. Neck flexion produces involuntary hip and knee flexion. Fundoscopic examination reveals blurring of the optic disc margins bilaterally. He has a medical history of type 2 diabetes mellitus and hypertension.

Arrange the following next steps in the management of this patient in the most appropriate chronological order.

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Answer

The correct sequence is to first obtain blood cultures, immediately administer empiric antibiotics and dexamethasone, then obtain a CT scan of the head, followed by a lumbar puncture, and finally tailor therapy based on culture results.

The correct chronological sequence begins with obtaining blood cultures to maximize diagnostic yield. Because this patient has papilledema and altered mental status, a CT scan of the head is required before a lumbar puncture to avoid herniation. Crucially, empiric antibiotics (including ampicillin for Listeria in a patient >50 years old) and dexamethasone must be administered immediately after blood cultures and MUST NOT be delayed while waiting for the CT scan. Once the CT scan confirms safety, the lumbar puncture is performed. Finally, therapy is tailored once culture and susceptibility results return.

Step-by-Step Solution

1

Identify the clinical syndrome and its complications.

The patient has signs of bacterial meningitis (fever, headache, nuchal rigidity/Brudzinski sign) complicated by signs of increased intracranial pressure (papilledema, lethargy).

Recognizing elevated intracranial pressure dictates whether a lumbar puncture can be performed safely or if neuroimaging is required first.

2

Determine the safety of an immediate lumbar puncture.

Lumbar puncture must be delayed until a CT scan of the head rules out a mass effect.

Performing a lumbar puncture in the setting of papilledema and altered mental status carries a high risk of fatal cerebral herniation.

3

Sequence the interventions to prevent treatment delays.

Blood cultures must be drawn STAT, followed immediately by empiric antibiotics and dexamethasone, before sending the patient for the CT scan.

Delaying antibiotics for neuroimaging or cerebrospinal fluid collection significantly increases morbidity and mortality in bacterial meningitis. Blood cultures can be drawn in minutes and should precede antibiotics.

Key Concept

In patients with suspected bacterial meningitis who have indications for a CT scan before lumbar puncture (e.g., papilledema, altered mental status, new-onset seizure, focal neurologic deficit), blood cultures and empiric antibiotics must be administered immediately and never delayed for neuroimaging.

Question 1766Question →

A patient with metastatic squamous cell carcinoma of the lung is admitted to the medical ward with profound lethargy, severe abdominal pain, and intractable vomiting. Initial laboratory analysis reveals a critically elevated serum calcium concentration of 15.2 mg/dL. Arrange the following therapeutic interventions in the standard sequential order of management, beginning with immediate acute stabilization, progressing through medical therapies based on their onset of action, and concluding with the definitive treatment for severe hypercalcemia if the patient subsequently develops anuric acute renal failure.

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Answer

The correct sequence is: 1) Intravenous isotonic saline, 2) Calcitonin, 3) Intravenous bisphosphonate, 4) Hemodialysis.

The correct management algorithm for severe hypercalcemia (greater than 14 mg/dL) prioritizes immediate stabilization followed by targeted pharmacotherapy. Severe hypercalcemia induces profound dehydration via nephrogenic diabetes insipidus, making aggressive isotonic saline the critical first step. Because bisphosphonates (the definitive medical treatment) take 48 to 72 hours to act, calcitonin is administered concurrently to rapidly reduce calcium levels within 4 to 6 hours. Zoledronic acid or pamidronate then provides sustained osteoclast inhibition. Finally, if the patient develops anuric acute renal failure and cannot tolerate aggressive hydration, hemodialysis becomes the necessary definitive treatment of last resort.

Step-by-Step Solution

1

Initiate aggressive volume resuscitation with intravenous isotonic saline.

Intravascular volume is restored.

Severe hypercalcemia causes nephrogenic diabetes insipidus and profound dehydration; intravenous hydration dilutes serum calcium and promotes its renal excretion.

2

Administer calcitonin.

Serum calcium levels decrease rapidly within 4 to 6 hours.

Provides an immediate, fast-acting bridge to lower calcium while waiting for definitive long-term therapies to take effect.

3

Administer an intravenous bisphosphonate.

Sustained inhibition of osteoclastic bone resorption is achieved.

Provides long-term, definitive control of malignant hypercalcemia, though its peak onset takes several days.

4

Perform hemodialysis with a low-calcium dialysate.

Mechanical removal of calcium from the blood circulation.

Reserved as a last-resort intervention when standard therapies fail or when aggressive hydration is contraindicated due to anuric acute kidney injury.

Key Concept

Standard stepwise management algorithm for severe hypercalcemia of malignancy.

Question 1767Question →

A 24-year-old man with a history of type 1 diabetes mellitus is brought to the emergency department due to altered mental status, vomiting, and abdominal pain. His vital signs show a heart rate of 128/min and blood pressure of 90/60 mm Hg. Laboratory evaluation reveals a blood glucose of 650 mg/dL, an arterial pH of 7.15, a serum bicarbonate of 10 mEq/L, and a serum potassium of 4.5 mEq/L. A diagnosis of diabetic ketoacidosis (DKA) is made.

Arrange the following steps in the correct chronological order for the initial and ongoing management of this patient's condition.

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Answer

The correct sequence for the management of diabetic ketoacidosis is: 1) Administer a rapid intravenous bolus of 0.9% normal saline, 2) Initiate a continuous intravenous infusion of regular insulin, 3) Add 5% dextrose to the intravenous fluids, 4) Discontinue the intravenous insulin infusion and transition to subcutaneous insulin.

The management of DKA follows a strict sequential algorithm to optimize metabolic recovery and minimize complications. 1) High-volume isotonic saline is always the first step to restore vascular volume and renal perfusion. 2) Intravenous regular insulin is initiated only after confirming serum potassium is > 3.3 mEq/L (in this patient it is 4.5 mEq/L, so insulin can be safely started alongside potassium replacement). 3) Once blood glucose drops below 200-250 mg/dL, 5% dextrose is added to the IV fluids. This prevents hypoglycemia and allows the insulin infusion to be continued until the anion gap closes. 4) Finally, once the anion gap has closed and the patient can eat, subcutaneous insulin is administered, and the IV infusion is stopped 1-2 hours later.

Step-by-Step Solution

1

Restore intravascular volume

Administer 0.9% normal saline

Severe dehydration is universally present in DKA; initial fluid resuscitation restores perfusion and promotes renal glucose clearance.

2

Correct insulin deficiency and ketoacidosis

Start intravenous regular insulin

After confirming potassium is not dangerously low, insulin is given to stop ketogenesis and drive potassium and glucose into cells.

3

Prevent hypoglycemia during treatment

Add dextrose to intravenous fluids

Glucose levels fall faster than ketones clear. Dextrose must be added when glucose < 200-250 mg/dL to allow continued insulin therapy for resolving the acidosis.

4

Transition to maintenance therapy

Switch to subcutaneous insulin

Once the anion gap is closed and the patient can eat, subcutaneous insulin is started. Intravenous insulin is stopped 1-2 hours later to prevent rebound ketoacidosis.

Key Concept

Management algorithm for Diabetic Ketoacidosis (DKA)

Estimated Time:1m 30s

Question 1768Question →

Review the following arterial blood gas (ABG) profiles obtained on room air. Match each laboratory ABG profile with its most likely corresponding primary acid-base disorder.

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Answer

pH 7.25, PaCO2 60 matches with Respiratory acidosis. pH 7.30, PaCO2 30 matches with Metabolic acidosis. pH 7.50, PaCO2 40 matches with Metabolic alkalosis. pH 7.55, PaCO2 25 matches with Respiratory alkalosis.

Each arterial blood gas profile uniquely demonstrates one of the four primary simple acid-base disorders by pairing the pH directional shift (acidemia or alkalemia) with the corresponding primary derangement in either

PaCO_2

(respiratory) or bicarbonate (metabolic).

Step-by-Step Solution

1

Evaluate the first ABG profile (pH

7.25

,

PaCO_2

60

,

HCO_3^-

26

).

Matches with Respiratory acidosis.

A pH below

7.35

defines acidemia. The high

PaCO_2

matches the acidic direction, identifying it as the primary disorder.

2

Evaluate the second ABG profile (pH

7.30

,

PaCO_2

30

,

HCO_3^-

14

).

Matches with Metabolic acidosis.

A pH below

7.35

defines acidemia. The low

HCO_3^-

matches the acidic direction, while the low

PaCO_2

represents respiratory compensation.

3

Evaluate the third ABG profile (pH

7.50

,

PaCO_2

40

,

HCO_3^-

32

).

Matches with Metabolic alkalosis.

A pH above

7.45

defines alkalemia. The high

HCO_3^-

directly matches the alkalemic direction.

4

Evaluate the fourth ABG profile (pH

7.55

,

PaCO_2

25

,

HCO_3^-

22

).

Matches with Respiratory alkalosis.

A pH above

7.45

defines alkalemia. The low

PaCO_2

directly matches the alkalemic direction.

Key Concept

Basic Interpretation of Arterial Blood Gases

Question 1769Question →

A 68-year-old man is evaluated in the intensive care unit for decreased urine output. Three days ago, he was admitted with septic shock secondary to a perforated appendix and underwent an emergent exploratory laparotomy. Since admission, he has received broad-spectrum antibiotics and aggressive intravenous fluid resuscitation. Over the past 24 hours, his total urine output has been $300$ mL. His blood pressure is $108/64$ mmHg and his heart rate is $92$ /min. The patient appears euvolemic on physical examination.

Laboratory studies are as follows:

Serum studies
Sodium: $140$ mEq/L
Potassium: $5.0$ mEq/L
Chloride: $104$ mEq/L
Bicarbonate: $18$ mEq/L
Blood urea nitrogen: $72$ mg/dL
Creatinine: $3.5$ mg/dL (baseline: $1.0$ mg/dL)

Spot urine studies
Sodium: $60$ mEq/L
Creatinine: $50$ mg/dL

Urinalysis reveals muddy brown granular casts.

What is this patient's calculated fractional excretion of sodium ( $FE_{Na}$ ) expressed as a percentage?

Show answer & explanation

Answer: 3

Answer

3

The correct value is 3. This patient's clinical presentation of septic shock, prolonged hypotension, and muddy brown granular casts on urinalysis is highly suggestive of acute tubular necrosis (ATN), an intrinsic form of acute kidney injury. The fractional excretion of sodium (

FE_{Na}

) is a useful calculation to help differentiate prerenal causes of acute kidney injury from intrinsic causes. It is calculated using the formula:

FE_{Na} = \frac{U_{Na} \times S_{Cr}}{S_{Na} \times U_{Cr}} \times 100

. By substituting this patient's laboratory values into the formula, the calculation is:

\frac{60 \times 3.5}{140 \times 50} \times 100 = \frac{210}{7000} \times 100 = 3\%

. Because the renal tubular epithelial cells are damaged in ATN, they lose their ability to appropriately reabsorb sodium from the filtrate. This leads to an inappropriately elevated amount of sodium spilling into the urine, resulting in an

FE_{Na}

of

> 2\%

.

Step-by-Step Solution

1

Identify the standard formula used to calculate the fractional excretion of sodium.

FE_{Na} = \frac{U_{Na} \times S_{Cr}}{S_{Na} \times U_{Cr}} \times 100

This formula measures the percentage of sodium filtered by the kidney that is ultimately excreted in the urine, helping to distinguish prerenal disease from intrinsic kidney injury.

2

Extract the specific values needed from the patient's laboratory studies.

U_{Na} = 60

mEq/L,

S_{Cr} = 3.5

mg/dL,

S_{Na} = 140

mEq/L, and

U_{Cr} = 50

mg/dL

Accurate identification of the serum and urine values for sodium and creatinine is necessary to perform the calculation.

3

Calculate the numerator by multiplying urine sodium and serum creatinine.

60 \times 3.5 = 210

This represents the first half of the clearance ratio.

4

Calculate the denominator by multiplying serum sodium and urine creatinine.

140 \times 50 = 7000

This represents the second half of the clearance ratio.

5

Divide the numerator by the denominator and multiply by 100 to find the percentage.

(210 / 7000) \times 100 = 0.03 \times 100 = 3\%

This step converts the absolute fractional value into a percentage, as specifically requested by the question stem.

Key Concept

The fractional excretion of sodium (

FE_{Na}

) is calculated as

(U_{Na} \times S_{Cr}) / (S_{Na} \times U_{Cr}) \times 100

. An

FE_{Na}

greater than

2\%

is typically consistent with intrinsic acute kidney injury (such as acute tubular necrosis), whereas a value less than

1\%

suggests prerenal azotemia.

Question 1770Question →

A $64$ -year-old man presents to the emergency department with a $5$ -day history of profound fatigue, severe headache, and nausea. He has a history of metastatic melanoma and has been receiving combination immunotherapy with nivolumab and ipilimumab for the past $12$ weeks. His blood pressure is $96/60$ mm Hg, and his heart rate is $92$ /min. Physical examination reveals no focal neurologic deficits, no orthostatic blood pressure changes, and normal skin pigmentation.

Laboratory studies show:
Sodium: $130$ mEq/L
Potassium: $4.1$ mEq/L
Glucose: $65$ mg/dL
Thyroid-stimulating hormone (TSH): $0.2$ mIU/L (Normal: $0.4-4.2$ mIU/L)
Free T4: $0.5$ ng/dL (Normal: $0.9-1.7$ ng/dL)
Morning (8:00 AM) serum cortisol: $2.1$ $\mu$ g/dL (Normal: $5-25$ $\mu$ g/dL)

Magnetic resonance imaging (MRI) of the brain reveals diffuse enlargement of the pituitary gland with stalk thickening.

Which of the following is the most appropriate management strategy for this patient?

Initiate hydrocortisone therapy prior to starting levothyroxine

Initiate levothyroxine therapy immediately to address the severe hypothyroidism

Add fludrocortisone therapy to correct the hyponatremia and hypotension

Titrate future levothyroxine doses to maintain the serum TSH level within the normal reference range

Administer intravenous hypertonic saline to acutely correct the hyponatremia

Show answer & explanation

Answer: Initiate hydrocortisone therapy prior to starting levothyroxine

Answer

Initiate hydrocortisone therapy prior to starting levothyroxine

The correct strategy addresses panhypopituitarism caused by immune checkpoint inhibitor-induced hypophysitis. The patient has central adrenal insufficiency (low cortisol) and central hypothyroidism (inappropriately low TSH with low free T4). The cardinal rule of endocrinology in panhypopituitarism is that glucocorticoid replacement (hydrocortisone) MUST be initiated before thyroid hormone replacement. Thyroid hormone increases the basal metabolic rate and accelerates the hepatic clearance of cortisol. If levothyroxine is given first to a cortisol-deficient patient, it can precipitate a fatal acute adrenal crisis (severe hypotension, shock, and vascular collapse).

Step-by-Step Solution

1

Analyze the patient's history and imaging findings.

The patient is on immune checkpoint inhibitors (ipilimumab/nivolumab) and has MRI evidence of pituitary enlargement.

This highly suggests immune checkpoint inhibitor-induced hypophysitis, an autoimmune inflammation of the pituitary gland.

2

Interpret the laboratory findings in the context of pituitary disease.

Low TSH with low free T4 indicates central hypothyroidism. Low morning cortisol with normal potassium and lack of hyperpigmentation indicates central adrenal insufficiency.

Hypophysitis frequently causes panhypopituitarism, leading to concurrent deficiencies in multiple anterior pituitary hormones (ACTH and TSH).

3

Determine the physiological interaction between the required hormone replacements.

The patient requires both glucocorticoids (hydrocortisone) and thyroid hormone (levothyroxine). However, thyroid hormone increases the metabolic clearance of cortisol.

Administering levothyroxine first in a patient with profound cortisol deficiency will rapidly clear what little cortisol they have left, precipitating a life-threatening adrenal crisis.

4

Establish the correct management sequence.

Glucocorticoids must be administered first.

Hydrocortisone replacement stabilizes the patient's hemodynamics and prevents adrenal crisis before the metabolic rate is increased by levothyroxine.

Key Concept

In patients with central panhypopituitarism (e.g., secondary to hypophysitis), glucocorticoid replacement must ALWAYS precede thyroid hormone replacement to avoid precipitating an acute adrenal crisis.

Question 1771Question →

An occupational health physician evaluates a 50-year-old male employee who reports an eight-month history of worsening fatigue, generalized weakness, cold intolerance, and a significant decline in sexual function. He has no significant past medical history and takes no medications. Vital signs show a blood pressure of $110/70$ mm Hg and a heart rate of $54$ /min. Physical examination reveals somewhat pale, doughy skin, delayed relaxation phase of the deep tendon reflexes, and normal testicular volume. There is no goiter palpable. Laboratory evaluation reveals the following:

Laboratory Parameter	Patient Value	Reference Range
Serum sodium	$137$ mEq/L	$136-145$ mEq/L
Free thyroxine (T4)	$0.5$ ng/dL	$0.9-1.7$ ng/dL
Thyroid-stimulating hormone (TSH)	$1.9$ mU/L	$0.5-5.0$ mU/L
Total testosterone	$140$ ng/dL	$300-1000$ ng/dL
Luteinizing hormone (LH)	$1.1$ mIU/mL	$1.5-9.3$ mIU/mL
Prolactin	$45$ ng/mL	$2-18$ ng/mL

Which of the following is the most likely underlying mechanism for this patient's laboratory abnormalities?

Autoimmune destruction of thyroid follicular cells

Autonomous hypersecretion from pituitary lactotrophs

Decreased peripheral deiodination of thyroxine

Mass effect compressing functional pituitary tissue and the infundibulum

Primary gonadal failure with absent negative feedback

Show answer & explanation

Answer: Mass effect compressing functional pituitary tissue and the infundibulum

Answer

The most likely underlying mechanism is mass effect compressing functional pituitary tissue and the infundibulum.

This patient's laboratory panel demonstrates central hypothyroidism (low free T4, normal TSH) and central hypogonadism (low testosterone, normal LH). The anterior pituitary is failing to mount the appropriate trophic hormone response (high TSH and high LH) to low peripheral hormone levels. Additionally, the prolactin level is mildly elevated. Prolactin is uniquely under tonic inhibitory control by hypothalamic dopamine delivered via the pituitary stalk (infundibulum). A sellar mass (such as a nonfunctioning pituitary macroadenoma) compressing the infundibulum disrupts this dopaminergic delivery, leading to mild hyperprolactinemia (the 'stalk effect'), while simultaneously compressing functional anterior pituitary tissue to cause panhypopituitarism.

Step-by-Step Solution

1

Interpret the thyroid function tests.

Low free T4 accompanied by an inappropriately normal TSH.

To differentiate between primary (thyroid gland) and secondary (central/pituitary) hypothyroidism.

2

Evaluate the gonadal axis.

Low total testosterone with an inappropriately normal LH.

To determine if the hypogonadism is primary (testicular failure) or secondary (central failure).

3

Analyze the prolactin level.

Mildly elevated prolactin (

45

ng/mL).

To differentiate a primary prolactin-secreting adenoma (typically

>200

ng/mL if large enough to cause mass effect) from the 'stalk effect'.

4

Synthesize the findings.

Combined central hypothyroidism, central hypogonadism, and the stalk effect point to a compressive sellar mass.

To identify the single unifying pathophysiologic mechanism.

Key Concept

Central endocrinopathies present with low target-hormone levels and inappropriately normal or low trophic hormones; a compressive sellar mass can cause panhypopituitarism accompanied by mildly elevated prolactin due to disruption of dopaminergic inhibition (stalk effect).

Question 1772Question →

During a multi-disciplinary endocrine case conference, several unique presentations of calcium and phosphorus dysregulation are discussed. Match each of the following distinct laboratory profiles with its corresponding underlying pathophysiologic mechanism. (Reference ranges: Calcium 8.4-10.2 mg/dL, Phosphorus 2.5-4.5 mg/dL, Intact PTH 10-65 pg/mL, 24-hr urine calcium 100-300 mg/day)

Click a left item, then click its matching right item

Show answer & explanation

Answer

Profile 1 maps to autonomous parathyroid hormone overproduction. Profile 2 maps to inactivating mutation of the calcium-sensing receptor. Profile 3 maps to pathologic extrarenal 1-alpha-hydroxylase activity. Profile 4 maps to end-organ resistance to parathyroid hormone.

Each laboratory profile distinctly maps to a unique disturbance in calcium-phosphorus homeostasis based on physiological feedback loops. Profile 1 shows autonomous PTH action (calcium up, phos down, urine calcium high). Profile 2 shows an altered set-point for calcium sensing (calcium up, phos normal, urine calcium uniquely low). Profile 3 shows PTH-independent hypercalcemia driven by ectopic Vitamin D (calcium up, phos up, PTH down). Profile 4 shows failure of PTH at target tissues (calcium down, phos up, PTH sky-high despite normal kidney function).

Step-by-Step Solution

1

Analyze Profile 1 for primary derangement

Identified hypercalcemia, hypophosphatemia, and hypercalciuria with inappropriately elevated PTH.

This pattern is classic for primary hyperparathyroidism, where an autonomous source (like an adenoma) secretes PTH independently of negative feedback.

2

Analyze Profile 2 for distinguishing features from Profile 1

Identified hypercalcemia with unsuppressed PTH but very low 24-hour urine calcium.

Hypocalciuria differentiates familial hypocalciuric hypercalcemia (FHH) from primary hyperparathyroidism. FHH requires higher serum calcium to suppress PTH and excrete calcium due to a mutant receptor.

3

Analyze Profile 3 for PTH-independent hypercalcemia

Identified hypercalcemia and hyperphosphatemia with properly suppressed PTH and elevated active Vitamin D.

Ectopic 1-alpha-hydroxylase activity (e.g., in sarcoidosis) creates excess active Vitamin D, which increases GI absorption of both calcium and phosphorus, subsequently suppressing endogenous PTH.

4

Analyze Profile 4 for cause of hypocalcemia

Identified hypocalcemia and hyperphosphatemia despite extremely high PTH levels and normal renal function.

The kidneys are not responding to PTH (which would normally lower phosphate and raise calcium). This end-organ resistance is the defining feature of pseudohypoparathyroidism.

Key Concept

Differentiation of calcium homeostasis disorders requires integrating serum calcium, phosphorus, and PTH, along with adjunctive markers like 24-hour urinary calcium and Vitamin D levels.

Estimated Time:2m 0s

Question 1773Question →

A 28-year-old woman is evaluated in the hospital on the third day of admission for acute pyelonephritis. On admission, she presented with fever, chills, right-sided flank pain, and dysuria. She was started on empiric intravenous ceftriaxone. Today, she reports feeling much better and has been afebrile for the past 24 hours. Her right costovertebral angle tenderness has largely resolved. She is tolerating a regular diet.

Urine culture obtained on admission has grown > $10^5$ CFU/mL of *Klebsiella pneumoniae*. Antimicrobial susceptibility testing results are as follows:

Antimicrobial	Interpretation
Ampicillin	Resistant
Cefazolin	Susceptible
Ceftriaxone	Susceptible
Ciprofloxacin	Susceptible
Nitrofurantoin	Susceptible
Trimethoprim-sulfamethoxazole	Susceptible

Which of the following is the most appropriate next step in management?

Transition to oral ciprofloxacin

Transition to oral nitrofurantoin

Transition to oral ampicillin

Continue intravenous ceftriaxone

Add intravenous gentamicin

Show answer & explanation

Answer: Transition to oral ciprofloxacin

Answer

Transition to oral ciprofloxacin

The correct action is to transition the patient to oral ciprofloxacin. This patient with acute pyelonephritis has demonstrated significant clinical improvement after initial intravenous therapy. She is hemodynamically stable, has been afebrile for 24 hours, and is tolerating oral intake. These meet the standard clinical criteria for transitioning from intravenous to oral antibiotics, which reduces hospital length of stay and minimizes the risk of line-associated complications. Fluoroquinolones (such as ciprofloxacin or levofloxacin) and trimethoprim-sulfamethoxazole are the preferred oral agents for pyelonephritis because they achieve excellent bioavailability and high therapeutic concentrations in the renal parenchyma. Since the isolated pathogen is susceptible, oral ciprofloxacin is a highly appropriate choice to complete her treatment course.

Step-by-Step Solution

1

Assess the patient's clinical trajectory and criteria for transitioning to oral therapy.

The patient is afebrile, clinically improving, and tolerating oral intake, indicating readiness for transition from intravenous to oral antibiotics.

Continuing intravenous therapy when oral transition criteria are met increases hospital stay and complication risks.

2

Evaluate the urine culture susceptibility panel to select an appropriate oral agent.

The pathogen is susceptible to several oral options, including ciprofloxacin, nitrofurantoin, and trimethoprim-sulfamethoxazole.

A targeted, narrow-spectrum agent must be chosen based on in vitro susceptibility.

3

Filter the susceptible oral agents based on their pharmacokinetic ability to penetrate renal tissue.

Ciprofloxacin achieves excellent renal tissue penetration, whereas nitrofurantoin does not.

Treating upper urinary tract infections (pyelonephritis) requires an antibiotic that reaches high concentrations in the renal parenchyma.

Key Concept

Patients with acute pyelonephritis who are clinically improving and tolerating oral intake should be transitioned to oral antibiotics with high renal tissue penetration, such as fluoroquinolones (ciprofloxacin) or trimethoprim-sulfamethoxazole. Nitrofurantoin is contraindicated as it does not achieve adequate levels in the kidney.

Question 1774Question →

Match each clinical presentation of non-anion gap metabolic acidosis (NAGMA) to its primary pathophysiological mechanism.

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Show answer & explanation

Answer

The correct matches align each clinical presentation with its underlying pathophysiological mechanism: the diarrhea patient with extrarenal bicarbonate loss; the Sjögren syndrome patient with impaired distal hydrogen secretion (Type 1 RTA); the diabetic patient with aldosterone deficiency/resistance (Type 4 RTA); the multiple myeloma patient with generalized proximal tubular dysfunction (Type 2 RTA); and the inhalant abuse patient with the excretion of unmeasured organic acid metabolites.

Each clinical vignette provides classic clues for a specific etiology of non-anion gap metabolic acidosis. The correct matches perfectly align the clinical scenario (diarrhea, Type 1 RTA, Type 4 RTA, Type 2 RTA/Fanconi, and Toluene toxicity) with their respective pathophysiological mechanisms by systematically applying the principles of urine electrolytes, urine pH, and serum potassium evaluation.

Step-by-Step Solution

1

Analyze the common presentation.

All five patients present with a hyperchloremic, non-anion gap metabolic acidosis (NAGMA).

Establishing the presence of NAGMA narrows the differential diagnosis to primarily gastrointestinal bicarbonate losses and renal tubular acidoses (RTAs).

2

Evaluate the urine anion gap (UAG).

The patient with diarrhea has a negative UAG (

-20\ mEq/L

), whereas all other patients have a positive UAG.

A negative UAG indicates that the kidneys are appropriately synthesizing and excreting ammonium (

NH_4^+

) with chloride to excrete the daily acid load, pointing to an extrarenal (GI) cause of acidosis.

3

Differentiate the classic renal tubular acidoses (RTAs) based on potassium and urine pH.

The diabetic patient has hyperkalemia (Type 4 RTA). The Sjögren patient has hypokalemia and urine pH > 5.5 (Type 1 RTA). The myeloma patient has glycosuria and phosphaturia (Type 2 RTA / Fanconi syndrome).

Type 4 RTA is uniquely characterized by hyperkalemia. Type 1 and Type 2 RTAs present with hypokalemia, but only Type 1 RTA features a strict inability to acidify the urine below a pH of 5.5.

4

Identify the toxicological mimic among the presentations.

The patient with a history of inhalant abuse (toluene) presents with a positive UAG but a high urine osmolal gap.

Toluene metabolism produces hippurate, which is excreted with

Na^+

or

K^+

rather than

Cl^-

. This creates a falsely positive UAG despite intact

NH_4^+

production (which is revealed by calculating the urine osmolal gap).

Key Concept

Evaluation of Non-Anion Gap Metabolic Acidosis (NAGMA) using serum potassium, urine pH, urine anion gap, and specific clinical contexts.

Question 1775Question →

A 21-year-old collegiate athlete is brought to the emergency department due to acute confusion, tinnitus, and severe nausea for the past 24 hours. He reports repeatedly taking "handfuls of pain pills" over the last three days to manage a severe ankle sprain. The patient has also had multiple episodes of forceful, non-bilious emesis today. His temperature is 38.0°C (100.4°F), heart rate is 115/min, blood pressure is 112/68 mm Hg, and respiratory rate is 32/min.

Laboratory Test	Result	Reference Range
Sodium ( $Na^+$ )	140 mEq/L	136-145 mEq/L
Potassium ( $K^+$ )	3.4 mEq/L	3.5-5.0 mEq/L
Chloride ( $Cl^-$ )	92 mEq/L	98-106 mEq/L
Bicarbonate ( $HCO_3^-$ )	16 mEq/L	22-28 mEq/L

Arterial Blood Gas (Room Air)	Result	Reference Range
pH	7.50	7.35-7.45
$PaCO_2$	21 mm Hg	35-45 mm Hg
$PaO_2$	98 mm Hg	75-105 mm Hg

Based on these findings, which of the following best describes this patient's acid-base disturbance?

Primary respiratory alkalosis, primary elevated anion gap metabolic acidosis, and primary metabolic alkalosis

Primary elevated anion gap metabolic acidosis with appropriate respiratory compensation

Primary respiratory alkalosis with appropriate metabolic compensation

Primary respiratory alkalosis and primary elevated anion gap metabolic acidosis only

Primary elevated anion gap metabolic acidosis and primary metabolic alkalosis only

Show answer & explanation

Answer: Primary respiratory alkalosis, primary elevated anion gap metabolic acidosis, and primary metabolic alkalosis

Answer

Primary respiratory alkalosis, primary elevated anion gap metabolic acidosis, and primary metabolic alkalosis

This patient's presentation and lab values indicate a classic triple acid-base disorder. The pH of 7.50 and

PaCO_2

of 21 mm Hg establish a primary respiratory alkalosis (due to central hyperventilation from salicylate toxicity). The anion gap of 32 establishes a primary high anion gap metabolic acidosis. Finally, the delta-delta calculation (

\Delta AG = 20

; Expected

HCO_3^- = 24 - 20 = 4

mEq/L) reveals that the actual

HCO_3^-

of 16 mEq/L is inappropriately high, proving the existence of a concurrent primary metabolic alkalosis caused by gastric acid loss from severe vomiting.

Step-by-Step Solution

1

Analyze the pH to determine the predominant acid-base status.

The pH is 7.50, which indicates alkalemia.

Identifying the primary direction of the pH shift helps isolate at least one of the primary disorders. Alkalemia means a primary alkalosis (either respiratory or metabolic) must be present.

2

Evaluate the

PaCO_2

in the context of the alkalemia.

The

PaCO_2

is 21 mm Hg (normal 40 mm Hg).

A low

PaCO_2

in the setting of alkalemia confirms the presence of a primary respiratory alkalosis. This is classically seen in salicylate toxicity due to direct stimulation of the medullary respiratory center.

3

Calculate the Anion Gap (AG) to assess for hidden metabolic acidosis.

AG = Na^+ - (Cl^- + HCO_3^-) = 140 - (92 + 16) = 32

.

An AG greater than 12 confirms the presence of a primary elevated anion gap metabolic acidosis (caused by uncoupling of oxidative phosphorylation and accumulation of lactic/salicylic acid).

4

Apply Winter's formula to determine expected respiratory compensation for the metabolic acidosis.

Expected\ PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 16) + 8 \pm 2 = 32 \pm 2

mm Hg.

Since the actual

PaCO_2

(21 mm Hg) is significantly lower than the expected range (30-34 mm Hg), it confirms the primary respiratory alkalosis is an independent process, not just compensation.

5

Calculate the Delta Gap (

\Delta AG

) and expected

HCO_3^-

to check for a third disorder.

\Delta AG = Actual\ AG - Normal\ AG = 32 - 12 = 20

.

Expected\ HCO_3^- = 24 - 20 = 4

mEq/L.

If this were a pure high AG metabolic acidosis, the

HCO_3^-

should drop to 4 mEq/L. Because the actual

HCO_3^-

(16 mEq/L) is much higher than 4 mEq/L, there must be a concurrent process adding bicarbonate to the blood: a primary metabolic alkalosis due to severe vomiting.

Key Concept

Diagnosis of mixed triple acid-base disorders using the anion gap, Winter's formula, and the delta-delta ratio.

Question 1776Question →

Match the following patient laboratory profiles to the most likely underlying pathophysiologic mechanism of their calcium homeostasis abnormality.

Reference ranges:
Calcium: $8.4-10.2$ mg/dL
Intact PTH: $10-65$ pg/mL
Phosphorus: $2.5-4.5$ mg/dL
24-hour urine calcium: $100-300$ mg/day

Click a left item, then click its matching right item

Show answer & explanation

Answer

Profile 1 matches autonomous overproduction of PTH (Primary hyperparathyroidism). Profile 2 matches an inactivating mutation of the CaSR (Familial hypocalciuric hypercalcemia). Profile 3 matches ectopic secretion of a peptide that activates the PTH receptor (Humoral hypercalcemia of malignancy). Profile 4 matches decreased intestinal absorption of calcium (Secondary hyperparathyroidism).

Each laboratory profile distinctly matches a specific pathophysiologic mechanism of calcium imbalance. Primary hyperparathyroidism is characterized by autonomous PTH secretion causing hypercalcemia and hypercalciuria. FHH is caused by an inactivating CaSR mutation leading to hypercalcemia with paradoxical hypocalciuria. Humoral hypercalcemia of malignancy is driven by ectopic PTHrP, which suppresses endogenous PTH and causes hypophosphatemia. Finally, states of decreased intestinal calcium absorption (like vitamin D deficiency) lead to hypocalcemia and an appropriate compensatory rise in PTH (secondary hyperparathyroidism).

Step-by-Step Solution

1

Analyze the first laboratory profile (Calcium 11.2, PTH 85, Urine Ca 350).

The profile shows hypercalcemia, high PTH, and hypercalciuria.

This confirms PTH-dependent hypercalcemia. The high urine calcium differentiates primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH).

2

Analyze the second laboratory profile (Calcium 11.1, PTH 75, Urine Ca 45).

The profile shows hypercalcemia, high PTH, and significant hypocalciuria.

Hypercalcemia with an inappropriately unsuppressed PTH and very low urine calcium (<100 mg/day) strongly suggests FHH, caused by an inactivating CaSR mutation.

3

Analyze the third laboratory profile (Calcium 12.5, PTH 5, Phos 2.1).

The profile shows severe hypercalcemia, suppressed PTH, and hypophosphatemia.

The suppressed intact PTH indicates a PTH-independent cause of hypercalcemia. The low phosphorus suggests a PTH-like effect, characteristic of PTHrP secretion in humoral hypercalcemia of malignancy.

4

Analyze the fourth laboratory profile (Calcium 7.8, PTH 90, Urine Ca 60).

The profile shows hypocalcemia, high PTH, and hypocalciuria.

Hypocalcemia stimulates a compensatory rise in PTH (secondary hyperparathyroidism). This is most often driven by poor intestinal calcium absorption due to vitamin D deficiency.

Key Concept

Differential diagnosis of calcium homeostasis disorders based on serum calcium, intact parathyroid hormone, and 24-hour urinary calcium excretion.

Question 1777Question →

During a preoperative evaluation clinic visit, an internal medicine physician is reviewing the medication list of an individual scheduled for an elective total hip replacement. The patient's past medical history includes heart failure with reduced ejection fraction, type 2 diabetes mellitus, a mechanical aortic valve replacement, and severe osteoarthritis. Match each of the patient's chronic medications to the most appropriate perioperative management strategy based on current clinical guidelines.

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Show answer & explanation

Answer

Warfarin requires bridging with low-molecular-weight heparin; Lisinopril should be held the morning of surgery; Bisoprolol must be continued through the morning of surgery; Dapagliflozin must be stopped 3-4 days prior to surgery.

Optimal perioperative management requires tailoring the handling of chronic medications to balance the risks of underlying disease exacerbation against the risks of anesthesia and surgical stress. Beta-blockers are maintained to prevent ischemia; ACE inhibitors are held to prevent severe vasoplegia; SGLT2 inhibitors are stopped early to avoid ketoacidosis; and warfarin is bridged for high-thrombotic-risk conditions.

Step-by-Step Solution

1

Assess the thromboembolic risk associated with the patient's mechanical aortic valve.

The risk is categorized as high, necessitating continuous therapeutic anticoagulation.

Determines that warfarin cannot simply be stopped; it requires bridging with low-molecular-weight heparin starting 5 days prior to the procedure.

2

Evaluate the risk of intraoperative hemodynamic instability for the patient's cardiac medications.

Lisinopril poses a risk of vasoplegic hypotension, while stopping bisoprolol risks rebound ischemia.

Guides the decision to hold the ACE inhibitor on the morning of surgery while continuing the beta-blocker uninterrupted.

3

Identify unique perioperative risks for the patient's diabetic medication.

Dapagliflozin can cause euglycemic diabetic ketoacidosis in the setting of fasting and surgical stress.

Mandates the proactive discontinuation of the SGLT2 inhibitor 3 to 4 days before the operation.

Key Concept

Perioperative Medication Management

Estimated Time:1m 30s

Question 1778Question →

A $48$ -year-old male is evaluated during a routine health maintenance visit. Three years prior, he underwent a transsphenoidal surgical resection for a pituitary macroadenoma, followed by targeted radiotherapy. His daily medication regimen includes hydrocortisone, testosterone gel, and levothyroxine. Recently, the patient has been experiencing worsening fatigue, unintended weight gain, and severe constipation. Vital signs are within normal limits. On physical examination, the physician notes a delayed relaxation phase of the Achilles tendon reflex bilaterally.

Laboratory studies obtained today show:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$0.04$ $\mu$ U/mL	$0.4$ - $4.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.7$ ng/dL	$0.9$ - $1.7$ ng/dL

Which of the following is the most appropriate next step in the management of this patient's medication regimen?

Increase the levothyroxine dosage

Decrease the levothyroxine dosage

Maintain the current levothyroxine dosage

Discontinue levothyroxine and initiate methimazole

Add liothyronine to the medication regimen

Show answer & explanation

Answer: Increase the levothyroxine dosage

Answer

Increase the levothyroxine dosage

In patients with intact pituitary function, primary hypothyroidism is managed by titrating levothyroxine to normalize the TSH level. However, in patients with a history of pituitary disease, surgery, or cranial radiation, the hypothalamic-pituitary axis is compromised. Therefore, TSH secretion is impaired and cannot be used as a reliable indicator of thyroid status. In these patients (who have secondary or tertiary hypothyroidism), levothyroxine replacement must be monitored using serum free T4 levels, aiming for the middle to upper half of the normal reference range. This patient presents with classic signs and symptoms of hypothyroidism (fatigue, weight gain, constipation, delayed deep tendon reflexes) and has a subnormal free T4 level (

0.7

ng/dL). This clearly indicates that he is under-replaced and requires an increase in his levothyroxine dosage. The suppressed TSH (

0.04

\mu

U/mL) is an expected consequence of his prior pituitary macroadenoma and its treatment, and it should not guide therapy.

Step-by-Step Solution

1

Identify the underlying etiology of the patient's thyroid dysfunction.

The patient has a history of a pituitary macroadenoma treated with surgery and radiation, indicating secondary (central) hypothyroidism rather than primary thyroid disease.

Understanding the level of the hypothalamic-pituitary-thyroid (HPT) axis defect is crucial because it dictates which laboratory values are reliable for monitoring therapy.

2

Analyze the current laboratory values and clinical symptoms in the context of the diagnosis.

The patient is experiencing classic symptoms of hypothyroidism (fatigue, weight gain, delayed reflexes) and has a subnormal free T4 level (

0.7

ng/dL), confirming inadequate thyroid hormone replacement.

Clinical status and free T4 levels are the primary parameters for assessing thyroid replacement adequacy in patients with central hypothyroidism.

3

Determine the appropriate management intervention based on the clinical assessment.

The levothyroxine dosage must be increased to raise the free T4 level into the middle to upper half of the normal reference range and to resolve the hypothyroid symptoms.

Because the pituitary gland is damaged, TSH secretion is permanently impaired. The suppressed TSH (

0.04

\mu

U/mL) is an expected artifact of the pituitary disease and must be ignored when titrating levothyroxine.

Key Concept

In patients with secondary (central) hypothyroidism due to pituitary or hypothalamic disease, levothyroxine replacement therapy must be monitored by measuring serum free T4 levels rather than TSH. The goal is to maintain the free T4 in the middle to upper half of the normal reference range.

Question 1779Question →

A 48-year-old woman is evaluated in the clinic for worsening tiredness, generalized weakness, and a 5-kg (11-lb) weight gain over the last eight months. She notes that her skin has become unusually dry, and she frequently feels cold even when others are comfortable. Her medical history is unremarkable, and she takes no medications. Vital signs are within normal limits except for a heart rate of 56/min. Physical examination reveals a diffusely enlarged, rubbery, and non-tender thyroid gland. Laboratory testing shows:

Serum TSH: 18.5 $\mu$ U/mL (Normal: 0.4-4.2)
Serum free T4: 0.5 ng/dL (Normal: 0.8-2.7)

Which of the following is the most likely underlying mechanism for this patient's clinical presentation?

Autoimmune-mediated destruction of thyroid follicular cells

Production of stimulating autoantibodies against the thyrotropin receptor

Decreased secretion of thyrotropin from the anterior pituitary gland

Peripheral resistance to circulating thyroid hormones

Granulomatous inflammation following a viral upper respiratory infection

Show answer & explanation

Answer: Autoimmune-mediated destruction of thyroid follicular cells

Answer

Autoimmune-mediated destruction of thyroid follicular cells

The correct answer accurately describes the pathophysiology of Hashimoto thyroiditis, which is the most common cause of primary hypothyroidism in iodine-sufficient areas. The patient's presentation of weight gain, fatigue, cold intolerance, bradycardia, and a diffusely enlarged, non-tender goiter is a classic clinical picture. The laboratory findings of an elevated TSH and a low free T4 confirm primary thyroid gland failure due to autoimmune-mediated destruction of the thyroid follicular cells.

Step-by-Step Solution

1

Interpret the thyroid function tests.

The patient has an elevated TSH and a decreased free T4 level.

To determine the thyroid functional status and localize the defect.

2

Diagnose the condition based on labs and symptoms.

The combination of high TSH, low free T4, weight gain, fatigue, bradycardia, and cold intolerance indicates primary hypothyroidism.

Elevated TSH with low T4 confirms the thyroid gland itself is the primary source of failure.

3

Identify the most common etiology.

Hashimoto thyroiditis (autoimmune-mediated destruction) is the most common cause of primary hypothyroidism in iodine-sufficient regions.

The presence of a painless, rubbery goiter strongly supports this autoimmune etiology.

Key Concept

Primary hypothyroidism is characterized by an elevated TSH and low free T4, most commonly due to autoimmune Hashimoto thyroiditis.

Estimated Time:1m 0s

Question 1780Question →

Match each of the following clinical conditions with its classic initial presentation of serum laboratory findings (calcium, phosphorus, and intact parathyroid hormone [PTH]).

Click a left item, then click its matching right item

Show answer & explanation

Answer

Primary hyperparathyroidism matches with elevated calcium, decreased phosphorus, elevated PTH. Hypoparathyroidism matches with decreased calcium, elevated phosphorus, decreased PTH. Chronic kidney disease matches with decreased calcium, elevated phosphorus, elevated PTH. Vitamin D deficiency matches with decreased calcium, decreased phosphorus, elevated PTH.

Calcium and phosphorus levels are primarily regulated by PTH and vitamin D. Understanding the primary defect allows prediction of the compensatory response and the resulting laboratory pattern. Primary hyperparathyroidism is driven by autonomous PTH excess (high Ca, low P). Hypoparathyroidism is defined by a lack of PTH (low Ca, high P). Chronic kidney disease features primary phosphorus retention and impaired vitamin D activation (low Ca, high P, high PTH). Vitamin D deficiency features poor gut absorption with intact renal function (low Ca, low P, high PTH).

Step-by-Step Solution

1

Analyze the laboratory profile for primary hyperparathyroidism.

Autonomous PTH production raises calcium and lowers phosphorus.

Excess PTH acts on the kidneys to reabsorb calcium and excrete phosphorus, and acts on bone to release calcium.

2

Analyze the laboratory profile for hypoparathyroidism.

Absent or low PTH results in low calcium and high phosphorus.

Without adequate PTH, the kidneys cannot adequately reabsorb calcium or excrete the daily phosphorus load.

3

Analyze the laboratory profile for chronic kidney disease.

Renal failure causes phosphorus retention and low 1,25-dihydroxyvitamin D, resulting in low calcium, high phosphorus, and compensatory high PTH.

The failing kidney cannot excrete phosphorus or properly activate vitamin D, leading to secondary hyperparathyroidism.

4

Analyze the laboratory profile for vitamin D deficiency.

Poor intestinal absorption leads to low calcium and low phosphorus, with a compensatory high PTH.

Secondary hyperparathyroidism in response to low calcium attempts to normalize serum calcium but simultaneously wastes phosphorus in the urine.

Key Concept

Calcium and Phosphorus Homeostasis

Internal Medicine

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