A 32-year-old nulliparous woman comes to the office due to a 10-month history of amenorrhe… — USMLE Step 2 CK

A $32$ -year-old nulliparous woman comes to the office due to a $10$ -month history of amenorrhea and an inability to conceive. She and her partner have been having regular, unprotected intercourse for $1.5$ years. Her previously regular $28$ -day menstrual cycles became increasingly irregular two years ago and ceased entirely $10$ months ago. The patient reports a recent onset of daily headaches, worsening fatigue, a $5$ -kg ( $11$ -lb) weight gain, and cold intolerance. Over the past few months, she has also noticed a bilateral, non-bloody, milky nipple discharge. She takes no medications. Vital signs are within normal limits. Physical examination reveals delayed relaxation of deep tendon reflexes and bilateral bitemporal hemianopsia on visual field testing.

Laboratory results:
Serum $\beta$ -hCG: Negative
Prolactin: $65$ ng/mL (Normal: $4$ - $23$ ng/mL)
Thyroid-stimulating hormone (TSH): $0.2$ $\mu$ U/mL (Normal: $0.4$ - $4.0$ $\mu$ U/mL)
Free T4: $0.5$ ng/dL (Normal: $0.9$ - $1.7$ ng/dL)
Follicle-stimulating hormone (FSH): $2$ mIU/mL (Normal: $4$ - $30$ mIU/mL)
Luteinizing hormone (LH): $1.5$ mIU/mL (Normal: $4$ - $30$ mIU/mL)

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

A
Hypothalamic thyrotropin-releasing hormone cross-stimulation of lactotrophs
Disruption of dopaminergic pathways in the infundibular stalkAnswer
C
Autonomous hypersecretion of prolactin by a pituitary macroadenoma
D
Autonomous hypersecretion of prolactin by a pituitary microadenoma
E
Dysregulation of gonadotropin secretion secondary to chronic hyperandrogenism

Answer

Disruption of dopaminergic pathways in the infundibular stalk

This patient's presentation of secondary amenorrhea, galactorrhea, central hypothyroidism (low TSH and low free T4), and bitemporal hemianopsia points to a large sellar or suprasellar mass. The mild elevation in prolactin (

65

ng/mL) in the presence of a large compressive mass is the hallmark of the 'stalk effect.' A non-functioning macroadenoma compresses the infundibular stalk, preventing hypothalamic dopamine (prolactin-inhibiting factor) from reaching the normal lactotrophs via the hypophyseal portal system. The loss of tonic dopamine inhibition results in mild hyperprolactinemia, which subsequently suppresses GnRH and causes amenorrhea and infertility. If this were a true macroprolactinoma, the prolactin level would correlate with the large tumor size and typically be massively elevated (

>200

ng/mL).

Step-by-Step Solution

Interpret the clinical presentation and physical exam findings.

The patient has secondary amenorrhea, galactorrhea, symptoms of hypothyroidism (fatigue, delayed reflexes), and bitemporal hemianopsia.

To localize the lesion; bitemporal hemianopsia strongly suggests a mass compressing the optic chiasm (a suprasellar/sellar mass).

Analyze the thyroid panel to determine the etiology of the hypothyroidism.

The free T4 is low (

0.5

ng/dL), and the TSH is inappropriately low (

0.2

\mu

U/mL).

To differentiate primary hypothyroidism (which would have an elevated TSH) from central hypothyroidism (caused by pituitary or hypothalamic dysfunction).

Evaluate the prolactin level in the context of the compressive mass.

Prolactin is mildly elevated at

65

ng/mL.

To distinguish between a prolactin-secreting macroadenoma (which typically secretes prolactin

>200

ng/mL) and a non-functioning mass.

Synthesize the findings to identify the underlying pathophysiologic mechanism.

A non-functioning pituitary macroadenoma is causing panhypopituitarism and compressing the infundibular stalk, preventing dopamine from reaching and inhibiting normal lactotrophs.

This 'stalk effect' unifies the central hypothyroidism, bitemporal hemianopsia, and mild hyperprolactinemia into a single diagnosis.

Key Concept

The 'stalk effect' occurs when a non-functioning sellar or suprasellar mass compresses the infundibular stalk, obstructing the delivery of hypothalamic dopamine to the anterior pituitary and resulting in mild hyperprolactinemia (

<200

ng/mL) along with symptoms of hypopituitarism.

Estimated Time:2m 30s

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