A physician is evaluating a 35-year-old female patient during a routine follow-up visit. S… — USMLE Step 2 CK

A physician is evaluating a $35$ -year-old female patient during a routine follow-up visit. She reports a $6$ -month history of debilitating fatigue, chronic constipation, and generalized dry skin. The patient's last menstrual period occurred $18$ months ago, just prior to the delivery of her first child. Her medical history is significant for a complicated vaginal delivery that required an emergent blood transfusion due to severe postpartum hemorrhage. She currently takes no medications. Vital signs show a blood pressure of $96/60$ mmHg, a pulse of $54$ /min, and respirations of $12$ /min. Physical examination reveals delayed relaxation of deep tendon reflexes and loss of axillary and pubic hair. Laboratory studies reveal the following:

Thyroid-stimulating hormone (TSH): $1.8$ $\mu$ U/mL (Normal: $0.4$ - $4.0$ $\mu$ U/mL)
Free thyroxine (T4): $0.5$ ng/dL (Normal: $0.9$ - $1.7$ ng/dL)

Which of the following additional laboratory findings is most likely to be present in this patient?

Low serum sodium concentrationAnswer
B
Elevated serum potassium concentration
C
Positive anti-thyroid peroxidase antibodies
D
Elevated serum prolactin level
E
Elevated serum follicle-stimulating hormone

Answer

Low serum sodium concentration

This patient's clinical presentation is classic for Sheehan syndrome, which is characterized by ischemic necrosis of the anterior pituitary gland following severe postpartum hemorrhage. The loss of anterior pituitary function leads to panhypopituitarism. Her low free T4 and inappropriately normal TSH indicate central hypothyroidism, and her amenorrhea indicates central hypogonadism. Furthermore, the loss of ACTH leads to secondary adrenal insufficiency, causing a profound deficiency in cortisol. Cortisol is required to suppress antidiuretic hormone (ADH) secretion; therefore, cortisol deficiency leads to increased ADH release and subsequent free water retention, presenting as dilutional hyponatremia (low serum sodium concentration).

Step-by-Step Solution

Interpret the thyroid laboratory studies.

The patient has a low free T4 with an inappropriately normal TSH, indicating central (secondary) hypothyroidism.

Primary hypothyroidism would present with an elevated TSH due to the loss of negative feedback.

Correlate the lab findings with the patient's clinical history.

The history of severe postpartum hemorrhage requiring a transfusion followed by amenorrhea and central hypothyroidism strongly suggests Sheehan syndrome (postpartum ischemic necrosis of the anterior pituitary).

Sheehan syndrome presents with panhypopituitarism, affecting multiple hormonal axes.

Determine the effects of panhypopituitarism on the adrenal axis.

The patient will have secondary adrenal insufficiency due to a lack of adrenocorticotropic hormone (ACTH), resulting in isolated cortisol deficiency with preserved aldosterone secretion.

Aldosterone is primarily regulated by the renin-angiotensin-aldosterone system (RAAS) and serum potassium, not ACTH.

Identify the expected electrolyte abnormalities in secondary adrenal insufficiency.

The patient is likely to develop hyponatremia without hyperkalemia.

Cortisol normally suppresses antidiuretic hormone (ADH) secretion. A lack of cortisol leads to increased ADH release, causing free water retention and dilutional hyponatremia. The preserved aldosterone prevents hyperkalemia.

Key Concept

Patients with Sheehan syndrome develop panhypopituitarism, including secondary adrenal insufficiency. The resulting cortisol deficiency leads to increased ADH secretion and dilutional hyponatremia, while intact RAAS function preserves aldosterone secretion, preventing hyperkalemia.

Estimated Time:1m 30s

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