An occupational health physician evaluates a 50-year-old male employee who reports an eigh… — USMLE Step 2 CK

An occupational health physician evaluates a 50-year-old male employee who reports an eight-month history of worsening fatigue, generalized weakness, cold intolerance, and a significant decline in sexual function. He has no significant past medical history and takes no medications. Vital signs show a blood pressure of $110/70$ mm Hg and a heart rate of $54$ /min. Physical examination reveals somewhat pale, doughy skin, delayed relaxation phase of the deep tendon reflexes, and normal testicular volume. There is no goiter palpable. Laboratory evaluation reveals the following:

Laboratory Parameter	Patient Value	Reference Range
Serum sodium	$137$ mEq/L	$136-145$ mEq/L
Free thyroxine (T4)	$0.5$ ng/dL	$0.9-1.7$ ng/dL
Thyroid-stimulating hormone (TSH)	$1.9$ mU/L	$0.5-5.0$ mU/L
Total testosterone	$140$ ng/dL	$300-1000$ ng/dL
Luteinizing hormone (LH)	$1.1$ mIU/mL	$1.5-9.3$ mIU/mL
Prolactin	$45$ ng/mL	$2-18$ ng/mL

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

A
Autoimmune destruction of thyroid follicular cells
B
Autonomous hypersecretion from pituitary lactotrophs
C
Decreased peripheral deiodination of thyroxine
Mass effect compressing functional pituitary tissue and the infundibulumAnswer
E
Primary gonadal failure with absent negative feedback

Answer

The most likely underlying mechanism is mass effect compressing functional pituitary tissue and the infundibulum.

This patient's laboratory panel demonstrates central hypothyroidism (low free T4, normal TSH) and central hypogonadism (low testosterone, normal LH). The anterior pituitary is failing to mount the appropriate trophic hormone response (high TSH and high LH) to low peripheral hormone levels. Additionally, the prolactin level is mildly elevated. Prolactin is uniquely under tonic inhibitory control by hypothalamic dopamine delivered via the pituitary stalk (infundibulum). A sellar mass (such as a nonfunctioning pituitary macroadenoma) compressing the infundibulum disrupts this dopaminergic delivery, leading to mild hyperprolactinemia (the 'stalk effect'), while simultaneously compressing functional anterior pituitary tissue to cause panhypopituitarism.

Step-by-Step Solution

Interpret the thyroid function tests.

Low free T4 accompanied by an inappropriately normal TSH.

To differentiate between primary (thyroid gland) and secondary (central/pituitary) hypothyroidism.

Evaluate the gonadal axis.

Low total testosterone with an inappropriately normal LH.

To determine if the hypogonadism is primary (testicular failure) or secondary (central failure).

Analyze the prolactin level.

Mildly elevated prolactin (

45

ng/mL).

To differentiate a primary prolactin-secreting adenoma (typically

>200

ng/mL if large enough to cause mass effect) from the 'stalk effect'.

Synthesize the findings.

Combined central hypothyroidism, central hypogonadism, and the stalk effect point to a compressive sellar mass.

To identify the single unifying pathophysiologic mechanism.

Key Concept

Central endocrinopathies present with low target-hormone levels and inappropriately normal or low trophic hormones; a compressive sellar mass can cause panhypopituitarism accompanied by mildly elevated prolactin due to disruption of dopaminergic inhibition (stalk effect).

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