A 48-year-old man comes to the physician due to progressive fatigue, feeling cold all the … — USMLE Step 2 CK

A 48-year-old man comes to the physician due to progressive fatigue, feeling cold all the time, and erectile dysfunction over the past 8 months. He also reports recent difficulty seeing cars in his peripheral vision while driving. He has a history of major depressive disorder, for which he took a selective serotonin reuptake inhibitor in the past, but he discontinued all medications two years ago. He currently takes no medications. Vital signs are within normal limits. Physical examination reveals delayed relaxation phases of the deep tendon reflexes and a lack of axillary hair. Visual field testing confirms bitemporal hemianopsia.

Laboratory studies are as follows:

Laboratory Study	Result	Reference Range
Sodium	$137\text{ mEq/L}$	$136-145\text{ mEq/L}$
Potassium	$4.2\text{ mEq/L}$	$3.5-5.0\text{ mEq/L}$
Morning cortisol	$4\text{ \mu g/dL}$	$5-23\text{ \mu g/dL}$
ACTH	$8\text{ pg/mL}$	$10-60\text{ pg/mL}$
TSH	$1.8\text{ \mu U/mL}$	$0.5-5.0\text{ \mu U/mL}$
Free T4	$0.5\text{ ng/dL}$	$0.9-1.7\text{ ng/dL}$
Prolactin	$52\text{ ng/mL}$	$< 20\text{ ng/mL}$
Testosterone	$120\text{ ng/dL}$	$300-1000\text{ ng/dL}$
LH	$1.0\text{ mIU/mL}$	$1.5-9.3\text{ mIU/mL}$

An MRI of the brain reveals a $2.5\text{ cm}$ sellar mass with suprasellar extension.

Which of the following is the most likely mechanism for the elevated serum prolactin level in this patient?

A
Autonomous secretion of hormone by proliferating lactotroph cells
B
Adverse effect of a psychotropic medication on the central nervous system
Disruption of dopaminergic pathways to the anterior pituitaryAnswer
D
Thyrotropin-releasing hormone-mediated lactotroph stimulation
E
Peripheral conversion of adrenal androgens into estrogenic compounds

Answer

Disruption of dopaminergic pathways to the anterior pituitary

This patient's clinical and laboratory findings indicate a nonfunctioning pituitary macroadenoma causing panhypopituitarism. The hallmark of a nonfunctioning sellar mass is the 'stalk effect.' Under normal physiological conditions, the hypothalamus tonically inhibits prolactin secretion from the anterior pituitary lactotrophs via the continuous release of dopamine down the infundibulum (pituitary stalk). When a large mass compresses the stalk, it disrupts this dopaminergic pathway. Without dopamine-mediated inhibition, the lactotrophs autonomously secrete prolactin, resulting in mild to moderate hyperprolactinemia (typically

< 200\text{ ng/mL}

Step-by-Step Solution

Analyze the clinical presentation and imaging findings.

The patient has bitemporal hemianopsia and an MRI showing a large (

2.5\text{ cm}

) sellar mass with suprasellar extension.

Identifying the structural lesion establishes the anatomical basis for the patient's symptoms (optic chiasm compression).

Interpret the comprehensive endocrine laboratory panel.

Low morning cortisol with low-normal ACTH, low free T4 with normal TSH, and low testosterone with low LH indicate panhypopituitarism (central failure of the adrenal, thyroid, and gonadal axes).

Determining whether the hormone deficiencies are primary (target gland) or secondary (pituitary) is essential for accurate diagnosis.

Evaluate the mechanism behind the isolated prolactin elevation.

A prolactin level of

52\text{ ng/mL}

is mildly elevated. Given the large mass, a true prolactinoma would cause much higher levels. The elevation is therefore due to compression of the pituitary stalk (stalk effect).

This differentiates a nonfunctioning pituitary macroadenoma from a prolactin-secreting macroadenoma, directly guiding management.

Key Concept

Stalk effect in nonfunctioning pituitary macroadenomas

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