A 52-year-old man is evaluated in the clinic for an 8-month history of worsening fatigue, … — USMLE Step 2 CK

A 52-year-old man is evaluated in the clinic for an $8$ -month history of worsening fatigue, decreased libido, and a $5\text{ kg}$ ( $11\text{ lb}$ ) weight gain. He works as a delivery driver and reports recently bumping into the side mirrors of parked cars because he "did not see them in the corners of his eyes." He has no significant past medical history and takes no medications. Vital signs show a heart rate of $54/\text{min}$ and blood pressure of $110/70\text{ mm Hg}$ . Physical examination reveals a puffy face and a delayed relaxation phase of the deep tendon reflexes. Visual field testing by confrontation confirms a loss of peripheral vision bilaterally.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.1\text{ mU/L}$	$0.4 - 4.0\text{ mU/L}$
Free thyroxine (T4)	$0.4\text{ ng/dL}$	$0.9 - 1.7\text{ ng/dL}$
Total testosterone	$120\text{ ng/dL}$	$300 - 1000\text{ ng/dL}$
Luteinizing hormone (LH)	$1.2\text{ mIU/mL}$	$1.5 - 9.3\text{ mIU/mL}$

Which of the following best describes the underlying pathophysiology of this patient's condition?

A
Autoimmune destruction of the thyroid gland follicular cells
B
Decreased peripheral tissue conversion of thyroxine to triiodothyronine
Compression of normal pituitary tissue by a space-occupying lesionAnswer
D
Primary failure of the Leydig cells with aberrant feedback signaling
E
Autoimmune-mediated destruction of the adrenal cortex

Answer

Compression of normal pituitary tissue by a space-occupying lesion

This patient's clinical presentation is classic for a non-functioning pituitary macroadenoma. He has symptoms of hypothyroidism (fatigue, weight gain, delayed reflex relaxation) and hypogonadism (decreased libido). The laboratory findings confirm central hypothyroidism (low Free T4 with an inappropriately normal TSH) and central hypogonadism (low testosterone with an inappropriately normal LH). Furthermore, his visual complaints describe bitemporal hemianopsia, a specific visual field defect caused by mass effect compressing the crossing nasal fibers at the optic chiasm. The underlying pathophysiology is the growth of a benign neoplasm (macroadenoma) that both compresses the optic chiasm superiorly and destroys or compresses the normal hormone-producing cells of the anterior pituitary, leading to panhypopituitarism.

Step-by-Step Solution

Interpret the clinical presentation and physical exam findings.

Fatigue, weight gain, bradycardia, and delayed reflexes suggest clinical hypothyroidism. Decreased libido suggests hypogonadism. Bumping into side mirrors indicates bitemporal hemianopsia (loss of peripheral vision).

To identify the distinct symptom clusters pointing toward specific anatomical and endocrine pathologies.

Analyze the thyroid laboratory panel.

The patient has a frankly low Free T4 with an inappropriately normal TSH.

To differentiate between primary hypothyroidism (where TSH would be elevated) and central hypothyroidism.

Evaluate the gonadal laboratory panel.

The patient has low total testosterone with an inappropriately normal/low LH.

To identify concurrent central hypogonadism, which, combined with central hypothyroidism, confirms panhypopituitarism.

Synthesize clinical and laboratory data to determine the etiology.

The combination of multiple anterior pituitary hormone deficiencies (panhypopituitarism) and optic chiasm compression (bitemporal hemianopsia) strongly indicates a pituitary macroadenoma.

To establish the underlying pathophysiological mechanism of the disease process.

Key Concept

Central hypothyroidism presents with low Free T4 and inappropriately normal or low TSH. When combined with other pituitary hormone deficiencies and bitemporal hemianopsia, it strongly suggests a compressive pituitary macroadenoma.

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