A 45-year-old man presents to the clinic complaining of profound fatigue, a 6-kg (13.2-lb)… — USMLE Step 2 CK

A 45-year-old man presents to the clinic complaining of profound fatigue, a 6-kg (13.2-lb) weight loss, and generalized weakness over the past 4 months. He also notes decreased libido and occasional dizziness when standing. Two years ago, he sustained a severe traumatic brain injury in a motor vehicle collision that required a 3-week stay in the intensive care unit. His blood pressure is 102/62 mm Hg while supine and 84/52 mm Hg upon standing. Heart rate is 86/min. Physical examination reveals loss of axillary and pubic hair, and delayed relaxation of deep tendon reflexes. Laboratory studies show:

* Sodium: $131 \text{ mEq/L}$
* Potassium: $4.0 \text{ mEq/L}$
* Thyroid-stimulating hormone (TSH): $2.2 \text{ } \mu\text{U/mL}$ (Normal: $0.4–4.0 \text{ } \mu\text{U/mL}$ )
* Free T4: $0.5 \text{ ng/dL}$ (Normal: $0.9–1.7 \text{ ng/dL}$ )

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

Measurement of early morning serum cortisolAnswer
B
Initiation of levothyroxine therapy
C
Measurement of thyroid peroxidase antibodies
D
Measurement of serum reverse T3
E
Magnetic resonance imaging of the brain

Answer

Measurement of early morning serum cortisol is the most appropriate next step to rule out concurrent central adrenal insufficiency before starting thyroid hormone replacement.

The correct answer addresses the critical need to evaluate for central adrenal insufficiency in a patient presenting with central hypothyroidism (indicated by low free T4 and normal TSH) following a traumatic brain injury. Administering levothyroxine before confirming and replacing cortisol in panhypopituitarism increases cortisol metabolism and can trigger a life-threatening adrenal crisis.

Step-by-Step Solution

Analyze the thyroid function tests.

The patient has a low free T4 but a normal TSH.

This discordant pattern (inappropriately normal TSH) indicates secondary (central) hypothyroidism rather than primary thyroid failure.

Integrate the clinical history and physical exam findings.

History of severe traumatic brain injury, fatigue, weight loss, orthostatic hypotension, hyponatremia, and loss of body hair.

These findings point to panhypopituitarism, including central adrenal insufficiency (explaining the hyponatremia and orthostasis without hyperkalemia) and hypogonadotropic hypogonadism.

Determine the sequence of management.

The hypothalamic-pituitary-adrenal axis must be evaluated before replacing thyroid hormone.

Levothyroxine increases the metabolic clearance of cortisol. If levothyroxine is given to a patient with untreated adrenal insufficiency, it can precipitate an acute, potentially fatal adrenal crisis. Therefore, an early morning serum cortisol level must be checked first.

Key Concept

In patients with central hypothyroidism (often part of panhypopituitarism), the hypothalamic-pituitary-adrenal axis must be evaluated with a serum cortisol level (and treated if necessary) before initiating levothyroxine therapy to prevent an adrenal crisis.

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