A 28-year-old woman is evaluated for a 12-month history of secondary amenorrhea and inabil… — USMLE Step 2 CK

A $28$ -year-old woman is evaluated for a $12$ -month history of secondary amenorrhea and inability to conceive. She had irregular menses since menarche and was diagnosed with polycystic ovary syndrome (PCOS) at age $21$ due to oligomenorrhea and hirsutism. Over the past year, her menses ceased entirely. She also reports progressive fatigue, cold intolerance, and occasional morning headaches. Her BMI is $32 \text{ kg/m}^2$ . Physical examination reveals coarse terminal hair on her chin and upper lip, unchanged from previous visits. Neurologic examination shows a visual field defect in the bilateral temporal fields.

Laboratory evaluation reveals:
Follicle-stimulating hormone: $2 \text{ mIU/mL}$ (normal: $4-30 \text{ mIU/mL}$ )
Luteinizing hormone: $1 \text{ mIU/mL}$ (normal: $2-20 \text{ mIU/mL}$ )
Prolactin: $58 \text{ ng/mL}$ (normal: $<20 \text{ ng/mL}$ )
Thyroid-stimulating hormone: $0.2 \text{ mIU/L}$ (normal: $0.4-4.0 \text{ mIU/L}$ )
Free thyroxine (T4): $0.4 \text{ ng/dL}$ (normal: $0.9-1.7 \text{ ng/dL}$ )

Which of the following is the most likely underlying cause of this patient's current amenorrhea?

A
Peripheral insulin resistance altering ovarian steroidogenesis
B
Lymphocytic infiltration and destruction of the thyroid gland
Compression of the pituitary stalk and functional anterior pituitary tissueAnswer
D
Autonomous proliferation of anterior pituitary lactotrophs
E
Partial deficiency of adrenal 21-hydroxylase enzyme

Answer

Compression of the pituitary stalk and functional anterior pituitary tissue

The correct answer is compression of the pituitary stalk and functional anterior pituitary tissue. This patient presents with bitemporal hemianopsia, hypogonadotropic hypogonadism (low FSH/LH, complete amenorrhea), and central hypothyroidism (low TSH, low free T4). Together, these findings strongly indicate a large sellar mass (macroadenoma) extending suprasellarly to compress both the optic chiasm and the normal pituitary gland. Her mild hyperprolactinemia (

58 \text{ ng/mL}

) is characteristic of the 'stalk effect.' Dopamine from the hypothalamus tonically inhibits prolactin release from lactotrophs. A large, nonfunctioning mass compresses the pituitary stalk, disrupting dopaminergic transport and resulting in mild prolactin elevation (typically

< 100-200 \text{ ng/mL}

). In contrast, true macroprolactinomas generally secrete prolactin at much higher levels, often

> 1000 \text{ ng/mL}

Step-by-Step Solution

Analyze the patient's neurological findings.

The patient has bitemporal hemianopsia and morning headaches, indicating a mass lesion near the optic chiasm, most likely a pituitary macroadenoma.

Identifying the structural lesion narrows the differential diagnosis to sellar or suprasellar masses.

Interpret the thyroid function tests.

TSH is low (

0.2 \text{ mIU/L}

) and Free T4 is low (

0.4 \text{ ng/dL}

), confirming central (secondary) hypothyroidism.

Differentiates pituitary/hypothalamic dysfunction from primary thyroid failure (which would have high TSH).

Interpret the gonadotropin and prolactin levels in the context of the sellar mass.

FSH and LH are low (hypogonadotropic hypogonadism). Prolactin is mildly elevated (

58 \text{ ng/mL}

) but less than

200 \text{ ng/mL}

The combination of hypopituitarism and mild hyperprolactinemia in the setting of a large mass points to stalk compression (loss of dopamine inhibition) by a nonfunctioning adenoma, rather than a true prolactinoma.

Key Concept

Nonfunctioning pituitary macroadenomas can present with mass effect (bitemporal hemianopsia), panhypopituitarism (central hypothyroidism, hypogonadotropic hypogonadism), and mild hyperprolactinemia due to pituitary stalk compression (stalk effect).

Estimated Time:1m 30s

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