A 39-year-old man comes to the clinic reporting a 6-month history of profound fatigue, dec… — USMLE Step 2 CK

A $39$ -year-old man comes to the clinic reporting a $6$ -month history of profound fatigue, decreased libido, and a $5\text{-kg}$ ( $11\text{-lb}$ ) weight gain. He has no prior medical history but was involved in a severe motorcycle collision $18$ months ago, sustaining a basilar skull fracture that required a $3$ -week intensive care unit stay. His blood pressure is $100/65\text{ mmHg}$ and heart rate is $56\text{/min}$ . Physical examination shows pale, dry skin, loss of axillary and pubic hair, and a delayed relaxation phase of the deep tendon reflexes. Laboratory results are as follows:

Test	Result	Reference Range
Serum sodium	$133 \text{ mEq/L}$	$136 - 145 \text{ mEq/L}$
Serum potassium	$4.0 \text{ mEq/L}$	$3.5 - 5.0 \text{ mEq/L}$
Thyroid-stimulating hormone (TSH)	$2.1 \text{ \mu U/mL}$	$0.4 - 4.0 \text{ \mu U/mL}$
Free T4	$0.5 \text{ ng/dL}$	$0.9 - 1.7 \text{ ng/dL}$

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

Measurement of 8 AM serum cortisolAnswer
B
Measurement of anti-thyroid peroxidase antibodies
C
Initiation of levothyroxine monotherapy
D
Administration of a thyrotropin-releasing hormone (TRH) stimulation test
E
Ultrasound of the thyroid gland

Answer

Measurement of 8 AM serum cortisol is the most appropriate next step to rule out concurrent central adrenal insufficiency before initiating thyroid replacement.

The correct next step is evaluating the adrenal axis with an 8 AM serum cortisol. This patient's laboratory profile (low Free T4, normal TSH) is diagnostic of central hypothyroidism, likely a delayed consequence of traumatic brain injury (TBI-induced hypopituitarism). Because TBI often causes multiple anterior pituitary hormone deficiencies, he is at high risk for concurrent central adrenal insufficiency. Thyroid hormone increases hepatic clearance of cortisol. Therefore, administering levothyroxine to a patient with unrecognized adrenal insufficiency can precipitate an acute, life-threatening adrenal crisis. Glucocorticoid deficiency must always be ruled out or treated prior to initiating thyroid hormone replacement in panhypopituitarism.

Step-by-Step Solution

Interpret the laboratory findings in the context of the patient's history.

The patient has a low Free T4 (

0.5 \text{ ng/dL}

) but a TSH that is inappropriately within the normal reference range (

2.1 \text{ \mu U/mL}

), rather than elevated.

This specific pattern points to a central (pituitary or hypothalamic) failure to produce TSH, rather than primary failure of the thyroid gland itself.

Correlate the biochemical pattern with the clinical presentation.

The history of severe traumatic brain injury, combined with signs of hypogonadism (loss of body hair, decreased libido) and mild hyponatremia, suggests panhypopituitarism.

Traumatic brain injury can cause shear stress on the pituitary stalk or direct ischemic injury to the gland, leading to multiple anterior pituitary hormone deficiencies.

Determine the necessary safety step before managing the hypothyroidism.

The adrenal axis must be evaluated (e.g., via 8 AM serum cortisol or a cosyntropin stimulation test) before giving thyroid hormone.

Thyroid hormone replacement increases basal metabolic rate and hepatic clearance of cortisol. If a patient has undiagnosed central adrenal insufficiency, administering levothyroxine first can rapidly deplete remaining cortisol stores and precipitate an acute adrenal crisis.

Key Concept

In patients with suspected central hypothyroidism (indicated by low Free T4 with inappropriately normal or low TSH), concurrent central adrenal insufficiency must be ruled out with an 8 AM cortisol or ACTH stimulation test before initiating levothyroxine therapy to avoid precipitating an adrenal crisis.

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