A 28-year-old man is brought to the outpatient clinic by his partner due to profound fatig… — USMLE Step 2 CK

A 28-year-old man is brought to the outpatient clinic by his partner due to profound fatigue, a $9\text{ kg}$ ( $19.8\text{ lb}$ ) weight gain, and depressive symptoms that have progressively worsened over the past 6 months. Nine months ago, the patient sustained a severe traumatic brain injury with a basilar skull fracture following a motorcycle collision, requiring a 3-week stay in the intensive care unit. He currently takes no medications. Vital signs show a blood pressure of $102/64\text{ mmHg}$ and a pulse of $54\text{/min}$ . Physical examination reveals dry skin, delayed relaxation of deep tendon reflexes, and diminished axillary and pubic hair. Visual field testing is normal.

Laboratory studies show:
* Thyroid-stimulating hormone (TSH): $0.9 \text{ \mu U/mL}$ (Reference: $0.4-4.2 \text{ \mu U/mL}$ )
* Free T4: $0.5 \text{ ng/dL}$ (Reference: $0.8-1.8 \text{ ng/dL}$ )
* Morning serum cortisol: $3.4 \text{ \mu g/dL}$ (Reference: $5-25 \text{ \mu g/dL}$ )

Which of the following is the most likely cause of this patient's abnormal laboratory findings?

Traumatic disruption of the hypothalamic-pituitary axisAnswer
B
Autoimmune destruction of thyroid follicular cells
C
Impaired peripheral conversion of thyroxine to triiodothyronine
D
Inadequate dietary intake of iodine
E
Antibody-mediated stimulation of the TSH receptor

Answer

Traumatic disruption of the hypothalamic-pituitary axis

The patient's laboratory results demonstrate a low free T4 with an inappropriately normal TSH. This lack of appropriate pituitary compensation is the hallmark of secondary (central) hypothyroidism. Additionally, the low morning cortisol level points to secondary adrenal insufficiency, indicating a broader panhypopituitarism. Severe traumatic brain injury can cause direct vascular damage to the anterior pituitary or shearing of the hypothalamic-pituitary stalk. This disruption leads to multiple pituitary hormone deficiencies that can become clinically apparent months to years after the initial trauma.

Step-by-Step Solution

Identify the clinical syndrome

The patient exhibits classic signs of hypothyroidism, including fatigue, weight gain, bradycardia, and delayed deep tendon reflexes.

Recognizing the symptom cluster narrows the differential to an endocrine deficiency.

Interpret the thyroid laboratory profile

The serum free T4 is low, but the TSH is inappropriately normal rather than elevated.

In primary hypothyroidism, the pituitary gland increases TSH secretion to compensate for low T4. An inappropriately normal or low TSH indicates a central (secondary) etiology.

Evaluate concurrent endocrine abnormalities

The morning serum cortisol is significantly low (

3.4 \text{ \mu g/dL}

A low morning cortisol alongside central hypothyroidism points to a broader failure of the anterior pituitary hormones (panhypopituitarism) rather than an isolated defect.

Synthesize with patient history

The history of a severe traumatic brain injury (TBI) 9 months prior links to the mechanism.

Severe shearing forces during a TBI can disrupt the hypothalamic-pituitary stalk or damage the pituitary parenchyma directly, leading to insidious onset of hypopituitarism.

Key Concept

Central (secondary) hypothyroidism presents with low free T4 and an inappropriately normal or low TSH. A history of severe head trauma should raise clinical suspicion for post-traumatic hypopituitarism.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic