A 56-year-old male patient is evaluated in the clinic for a 6-month history of progressive… — USMLE Step 2 CK

A $56$ -year-old male patient is evaluated in the clinic for a $6$ -month history of progressive fatigue, cold intolerance, and erectile dysfunction. He also notes that he has recently scraped the side of his car twice while pulling out of his garage, which he attributes to increasing clumsiness. His medical history is unremarkable, and he takes no medications. Blood pressure is $110/70$ mmHg and pulse is $56$ /min. Physical examination demonstrates a delayed relaxation phase of the deep tendon reflexes and bilateral loss of peripheral vision in the temporal fields. Laboratory studies show the following:

Sodium: $135$ mEq/L
Thyroid-stimulating hormone (TSH): $2.2$ $\mu$ U/mL (Normal: $0.4-4.2$ $\mu$ U/mL)
Free thyroxine (T4): $0.5$ ng/dL (Normal: $0.9-2.4$ ng/dL)
Total testosterone: $140$ ng/dL (Normal: $300-1000$ ng/dL)

Which of the following best describes the pathogenesis of this patient's thyroid laboratory abnormalities?

Mechanical compression of thyrotroph cells by an expanding sellar massAnswer
B
Lymphocytic infiltration and destruction of the thyroid gland
C
Insufficient dietary intake of iodine impairing hormone synthesis
D
Decreased hepatic production of thyroxine-binding globulin
E
Mutation in the thyroid hormone receptor beta gene

Answer

The correct pathogenesis is mechanical compression of thyrotroph cells by an expanding sellar mass, consistent with a pituitary macroadenoma.

The correct answer describes central (secondary) hypothyroidism caused by a pituitary macroadenoma. This patient presents with classic symptoms of hypothyroidism (fatigue, cold intolerance, delayed deep tendon reflexes) and hypogonadism (erectile dysfunction, low testosterone). Crucially, he also exhibits signs of bitemporal hemianopsia (frequently bumping into objects on his sides, bilateral loss of peripheral vision), which strongly indicates a mass lesion in the sella turcica compressing the optic chiasm. The laboratory findings confirm central hypothyroidism: despite a frankly low free T4 level, the TSH is inappropriately normal. In primary hypothyroidism, the low free T4 would trigger a loss of negative feedback, resulting in a markedly elevated TSH. A normal or low TSH in the setting of low free T4 points to a failure of the anterior pituitary to secrete adequate thyrotropin. The most common cause of this presentation in adults is a pituitary macroadenoma, which mechanically compresses the normal pituitary tissue (including thyrotroph and gonadotroph cells), leading to panhypopituitarism.

Step-by-Step Solution

Analyze the patient's symptoms and physical exam findings.

The patient has signs of hypothyroidism (fatigue, cold intolerance, delayed reflexes), hypogonadism (erectile dysfunction), and bitemporal hemianopsia (scraping the car sides, temporal visual field loss).

Identifying the clinical syndrome helps localize the lesion. Bitemporal hemianopsia strongly suggests a mass at the optic chiasm (sella turcica).

Interpret the thyroid function tests.

Free T4 is frankly low, but TSH is inappropriately normal.

In primary hypothyroidism, a low T4 triggers a massive increase in TSH. A low T4 with a normal/low TSH indicates central (secondary) hypothyroidism.

Synthesize the clinical and laboratory findings to determine the underlying cause.

The combination of panhypopituitarism (central hypothyroidism and hypogonadism) with a compressive visual symptom points to a pituitary macroadenoma.

An expanding sellar mass mechanically compresses the optic chiasm and the normal anterior pituitary tissue, impairing the secretion of multiple trophic hormones including TSH and LH/FSH.

Key Concept

Central (secondary) hypothyroidism is characterized by a low free T4 level with an inappropriately normal or low TSH level, often caused by a pituitary macroadenoma that mechanically compresses the anterior pituitary gland.

Estimated Time:1m 30s

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