A 42-year-old man comes to the clinic reporting a 4-month history of progressive fatigue, … — USMLE Step 2 CK

A 42-year-old man comes to the clinic reporting a 4-month history of progressive fatigue, generalized weakness, and decreased libido. He also notes occasional dull, morning headaches that improve as the day progresses. He has no significant past medical history and takes no medications. Vital signs show a heart rate of $56$ /min and a blood pressure of $105/65$ mm Hg. Physical examination reveals delayed relaxation of deep tendon reflexes. His testes are soft and measure $12$ mL bilaterally. Visual fields are full to confrontation. Laboratory studies show:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$2.4$ $\mu$ U/mL	$0.4-4.2$ $\mu$ 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?

Measure morning serum cortisol levelAnswer
B
Initiate oral levothyroxine
C
Measure serum thyroid peroxidase antibodies
D
Obtain an ultrasound of the thyroid gland
E
Schedule a follow-up for repeat thyroid function testing in 6 weeks

Answer

The correct answer is to measure a morning serum cortisol level to rule out concurrent central adrenal insufficiency before treating the central hypothyroidism.

The correct answer addresses the critical procedural rule in endocrinology: when a patient presents with central hypothyroidism (low free T4, inappropriately normal/low TSH), panhypopituitarism must be suspected. The adrenal axis must be evaluated with a morning serum cortisol level before initiating thyroid hormone replacement. Giving levothyroxine accelerates cortisol metabolism, which can precipitate acute adrenal crisis if the patient has concurrent untreated central adrenal insufficiency.

Step-by-Step Solution

Interpret the thyroid function tests.

The patient has a low free T4 level but an inappropriately normal TSH.

This specific pattern differentiates central (secondary) hypothyroidism from primary hypothyroidism (where TSH would be elevated).

Correlate laboratory findings with the clinical presentation.

The combination of central hypothyroidism, decreased libido with soft testes (suggesting central hypogonadism), and morning headaches points to panhypopituitarism caused by a pituitary mass.

Establishing the etiology ensures that all potentially affected endocrine axes are considered.

Determine the safest immediate management step.

Order a morning serum cortisol level prior to initiating levothyroxine therapy.

Administering thyroid hormone increases the metabolic clearance of cortisol. If concurrent adrenal insufficiency is present and unrecognized, starting levothyroxine can precipitate a fatal adrenal crisis.

Key Concept

In patients diagnosed with central hypothyroidism, it is essential to evaluate for concurrent central adrenal insufficiency (e.g., via 8:00 AM serum cortisol) before initiating levothyroxine to avoid precipitating an adrenal crisis.

Estimated Time:1m 30s

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