A 58-year-old man presents to the oncology clinic complaining of worsening fatigue, genera… — USMLE Step 2 CK

A $58$ -year-old man presents to the oncology clinic complaining of worsening fatigue, generalized weakness, and cold intolerance for the past month. He also reports intermittent nausea and a $3$ -kg ( $6.6$ -lb) weight loss during this time. The patient has a history of advanced renal cell carcinoma, for which he has been receiving nivolumab therapy for the past six months. Blood pressure is $96/60$ mm Hg, and pulse is $54$ /min. Physical examination reveals dry skin and a delayed relaxation phase of the deep tendon reflexes.

Laboratory studies show:
Thyroid-stimulating hormone (TSH): $1.1$ $\mu$ U/mL (Normal: $0.5 - 5.0$ $\mu$ U/mL)
Free thyroxine (T4): $0.4$ ng/dL (Normal: $0.9 - 2.4$ ng/dL)
Sodium: $131$ mEq/L
Potassium: $4.1$ mEq/L

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

Assessment of early morning serum cortisol and adrenocorticotropic hormone levelsAnswer
B
Immediate initiation of oral levothyroxine therapy
C
Assessment of serum anti-thyroid peroxidase antibody titers
D
Magnetic resonance imaging of the brain
E
Discontinuation of nivolumab and observation for spontaneous recovery

Answer

The most appropriate next step is the assessment of early morning serum cortisol and adrenocorticotropic hormone levels.

The correct answer involves assessing early morning serum cortisol and adrenocorticotropic hormone (ACTH) levels. This patient's laboratory results demonstrate a low free T4 with an inappropriately normal TSH, establishing the diagnosis of central (secondary) hypothyroidism. Given his history of receiving nivolumab, he most likely has immune checkpoint inhibitor (ICI)-induced hypophysitis. ICI-induced hypophysitis typically affects multiple anterior pituitary axes, with central adrenal insufficiency and central hypothyroidism being the most common and clinically significant. A critical principle in endocrinology is that the adrenal axis MUST be evaluated before treating central hypothyroidism. Thyroid hormone upregulates the metabolic clearance of cortisol. If levothyroxine is administered to a patient with unrecognized adrenal insufficiency, it can rapidly deplete the remaining circulating cortisol and precipitate a life-threatening adrenal crisis. Therefore, assessing cortisol and ACTH (or administering glucocorticoids empirically if the patient is in an acute shock state) is the mandatory next step.

Step-by-Step Solution

Analyze the thyroid function laboratory pattern.

The patient has a low free T4 (

0.4

ng/dL) with an inappropriately normal TSH (

1.1

\mu

U/mL).

This specific combination distinguishes central (secondary) hypothyroidism from primary hypothyroidism, where the TSH would be appropriately elevated.

Correlate the central hypothyroidism with the patient's pharmacological history and clinical presentation.

The patient is receiving nivolumab (an immune checkpoint inhibitor) and presents with fatigue, nausea, hypotension, and hyponatremia.

Immune checkpoint inhibitors are a major cause of autoimmune hypophysitis, leading to multiple anterior pituitary hormone deficiencies (panhypopituitarism).

Determine the critical management sequence for panhypopituitarism.

The adrenal axis must be evaluated (and treated if deficient) prior to initiating thyroid hormone replacement.

Thyroid hormone upregulates the hepatic clearance of cortisol. Administering levothyroxine to a patient with concurrent, untreated adrenal insufficiency can deplete remaining cortisol and precipitate a life-threatening adrenal crisis.

Key Concept

In patients with central hypothyroidism due to suspected hypopituitarism, concurrent central adrenal insufficiency must be ruled out or treated before initiating levothyroxine therapy to prevent an acute adrenal crisis.

Estimated Time:1m 30s

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