A 34-year-old woman comes to the general internal medicine clinic due to a four-month hist… — USMLE Step 2 CK

A $34$ -year-old woman comes to the general internal medicine clinic due to a four-month history of severe fatigue, generalized weakness, and a $5$ kg ( $11$ lb) unintended weight gain. She reports that she has not had a menstrual period since the birth of her second child $14$ months ago. Her medical records indicate that the delivery was complicated by severe hemorrhage necessitating the transfusion of four units of packed red blood cells. She is currently not taking any medications. Her blood pressure is $98/60$ mm Hg, and her pulse is $54$ /min. Physical examination demonstrates pale, dry skin and a delayed relaxation phase of the Achilles tendon reflexes.

Laboratory results are as follows:

Test	Result	Reference Range
Hemoglobin	$11.2$ g/dL	$12.0 - 15.5$ g/dL
Thyroid-stimulating hormone (TSH)	$0.3$ $\mu$ U/mL	$0.5 - 5.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.5$ ng/dL	$0.9 - 1.7$ ng/dL

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

A
Initiate oral levothyroxine monotherapy
Evaluate early morning serum cortisol and adrenocorticotropic hormoneAnswer
C
Obtain a diagnostic ultrasound of the thyroid gland
D
Measure serum thyroid-stimulating immunoglobulin
E
Recommend symptomatic treatment and repeat the thyroid panel in 6 weeks

Answer

Evaluate early morning serum cortisol and adrenocorticotropic hormone

The patient's history of postpartum hemorrhage followed by amenorrhea, fatigue, and weight gain is classic for Sheehan syndrome (ischemic necrosis of the pituitary gland). The laboratory findings of a low free T4 alongside an inappropriately low TSH confirm secondary (central) hypothyroidism. When central hypothyroidism is diagnosed, it is imperative to evaluate the adrenal axis (cortisol and ACTH) before administering thyroid hormone replacement. Levothyroxine increases the basal metabolic rate and enhances the hepatic clearance of glucocorticoids; if administered to a patient with concurrent, untreated adrenal insufficiency, it can precipitate a life-threatening acute adrenal crisis.

Step-by-Step Solution

Interpret the thyroid function tests

Low TSH and low free T4

To differentiate between primary and secondary (central) hypothyroidism

Correlate with clinical history

Suspect Sheehan syndrome (postpartum pituitary necrosis)

The patient has central hypothyroidism, amenorrhea, and a history of severe postpartum hemorrhage

Determine the necessary safety step before treating central hypothyroidism

Evaluate the adrenal axis

Initiating levothyroxine in a patient with unrecognized central adrenal insufficiency can precipitate an acute adrenal crisis

Key Concept

In patients with suspected secondary (central) hypothyroidism and potential panhypopituitarism, the adrenal axis must be evaluated before initiating levothyroxine to avoid precipitating an acute adrenal crisis.

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