A 52-year-old male presents to the outpatient clinic reporting a 6-month history of worsen… — USMLE Step 2 CK

A 52-year-old male presents to the outpatient clinic reporting a 6-month history of worsening fatigue, decreased libido, and erectile dysfunction. He also notes chronic constipation and feeling unusually sluggish. His medical history is unremarkable, and he takes no medications. Vital signs show a blood pressure of $110/70 \text{ mmHg}$ and a heart rate of $56/\text{min}$ . Physical examination reveals pale, dry skin and a delay in the relaxation phase of the deep tendon reflexes. Visual fields are full to confrontation.

Laboratory results are as follows:

Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.2 \text{ mIU/L}$	$0.5-5.0 \text{ mIU/L}$
Free thyroxine (T4)	$0.5 \text{ ng/dL}$	$0.9-1.7 \text{ ng/dL}$
Total testosterone	$140 \text{ ng/dL}$	$300-1000 \text{ ng/dL}$
Luteinizing hormone (LH)	$1.5 \text{ mIU/mL}$	$1.5-9.3 \text{ mIU/mL}$

Which of the following is the most likely underlying mechanism for this patient's presentation?

Compression of anterior pituitary tissue by a benign sellar massAnswer
B
Autoimmune lymphocytic infiltration of the thyroid gland
C
Fibrotic replacement of the testicular interstitium
D
Activating mutation of the thyrotropin receptor
E
Target organ resistance to circulating thyroid hormone

Answer

Compression of anterior pituitary tissue by a benign sellar mass

This patient's laboratory findings reveal concurrent central hypothyroidism (low free T4 with inappropriately normal TSH) and central hypogonadism (low testosterone with inappropriately normal LH). When multiple anterior pituitary axes fail simultaneously, it typically indicates a destructive or compressive lesion in the sella turcica. The most common cause of panhypopituitarism in adults is a nonfunctioning pituitary adenoma, which compresses the normal anterior pituitary tissue and impairs its ability to secrete trophic hormones.

Step-by-Step Solution

Analyze the thyroid laboratory values.

The patient has low free T4 (

0.5 \text{ ng/dL}

) with an inappropriately normal TSH (

1.2 \text{ mIU/L}

This pattern indicates secondary (central) hypothyroidism. In primary hypothyroidism, the normal pituitary would secrete high levels of TSH in response to low T4.

Analyze the gonadal laboratory values.

The patient has low total testosterone (

140 \text{ ng/dL}

) with an inappropriately normal LH (

1.5 \text{ mIU/mL}

This pattern indicates secondary (central) hypogonadism. In primary testicular failure, the LH would be elevated.

Synthesize the laboratory findings to identify the site of pathology.

Deficiencies in multiple anterior pituitary axes (panhypopituitarism) point to a central structural lesion.

The most common cause of acquired combined anterior pituitary hormone deficiencies in an adult male is compression of the glandular tissue by a pituitary adenoma (mass effect).

Key Concept

Central (Secondary) Endocrine Deficiencies

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