An ambulatory 29-year-old male construction worker presents to his primary care provider w… — USMLE Step 2 CK

An ambulatory 29-year-old male construction worker presents to his primary care provider with a 6-month history of progressive fatigue, diminished libido, and a 6-kg (13-lb) weight gain. He states that he feels constantly sluggish and cold, even during warm weather. Fourteen months ago, he sustained a severe traumatic brain injury during a scaffolding collapse at work, which required a 3-week stay in the intensive care unit. His blood pressure is 105/70 mm Hg, and his pulse is 55/min. Physical examination reveals pale skin, sparse axillary and pubic hair, and a delayed relaxation phase of the deep tendon reflexes.

Laboratory studies show:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.2\ \mu\text{U/mL}$	$0.5\text{ - }5.0\ \mu\text{U/mL}$
Free thyroxine (T4)	$0.3\text{ ng/dL}$	$0.9\text{ - }2.4\text{ ng/dL}$

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

Magnetic resonance imaging of the brainAnswer
B
Measurement of serum anti-thyroid peroxidase antibodies
C
Measurement of serum reverse triiodothyronine
D
Thyroid gland ultrasonography
E
Radioactive iodine uptake scan

Answer

Magnetic resonance imaging of the brain

Magnetic resonance imaging of the brain is the correct next step because this patient has central (secondary) hypothyroidism as part of a broader post-traumatic hypopituitarism syndrome. His clinical presentation includes features of hypothyroidism (weight gain, delayed reflexes) and hypogonadism (sparse hair, low libido). The diagnosis of central hypothyroidism is confirmed by a low free T4 paired with an inappropriately normal TSH. The pituitary gland failed to increase TSH secretion in response to the low free T4. Severe traumatic brain injury is a known cause of anterior pituitary dysfunction due to direct tissue contusion, shearing of the pituitary stalk, or vascular injury. Once central hypothyroidism is identified biochemically, patients must undergo neuroimaging (MRI) of the hypothalamic-pituitary region and have other pituitary axes evaluated.

Step-by-Step Solution

Analyze the clinical presentation and physical examination.

The patient exhibits classic signs of hypothyroidism (fatigue, cold intolerance, weight gain, bradycardia, delayed reflexes) and hypogonadism (diminished libido, sparse body hair) 14 months after a severe traumatic brain injury.

Identifying the multi-axis endocrine deficits provides context for interpreting the laboratory results.

Interpret the thyroid function laboratory studies.

The free T4 is overtly low (

0.3\ \text{ng/dL}

), but the TSH is inappropriately normal (

1.2\ \mu\text{U/mL}

A normal or low TSH in the presence of a low free T4 indicates a failure of the pituitary gland to mount an appropriate compensatory response.

Determine the underlying diagnosis and select the next appropriate step.

The laboratory pattern confirms central (secondary) hypothyroidism, likely caused by post-traumatic hypopituitarism. The next step is to obtain a brain MRI and evaluate other anterior pituitary hormones.

Anatomical imaging of the hypothalamic-pituitary axis is required to evaluate for structural damage (e.g., stalk shearing, ischemic necrosis, or empty sella) resulting from the prior trauma.

Key Concept

Central (secondary) hypothyroidism presents with a low free T4 level and an inappropriately normal or low TSH. Following a traumatic brain injury, patients can develop post-traumatic hypopituitarism; the diagnosis requires evaluation of pituitary hormones and magnetic resonance imaging of the brain.

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