A 38-year-old woman is evaluated in the outpatient clinic for generalized weakness, unremitting fatigue, and the absence of menstruation for the past 7 months. She also notes a spontaneous, bilateral milky discharge from her breasts. The patient has no prior medical conditions, takes no prescription medications, and a recent home pregnancy test was negative. Blood pressure is 110/70 mm Hg and pulse is 54/min. Physical examination reveals delayed relaxation of the deep tendon reflexes and pale conjunctivae. Visual field testing is normal.
Laboratory evaluation shows the following:
| Test | Result | Reference Range |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 1.8 mU/L | 0.4–4.0 mU/L |
| Free thyroxine () | 0.5 ng/dL | 0.9–1.7 ng/dL |
| Prolactin | 65 ng/mL | 4–23 ng/mL |
Which of the following is the most likely pathophysiologic mechanism explaining this patient's findings?
- ALymphocytic infiltration of the thyroid gland
- BAutonomous hypersecretion by anterior pituitary lactotrophs
- Compression of the infundibular stalkAnswer
- DIschemic infarction of the anterior pituitary gland
- EReduced tissue deiodination of circulating thyroxine
Answer
Compression of the infundibular stalk
The correct answer is compression of the infundibular stalk. This patient's laboratory results demonstrate a low free with an inappropriately normal TSH, definitively indicating central (secondary) hypothyroidism. Concurrently, she has mild hyperprolactinemia causing galactorrhea, and central hypogonadism causing amenorrhea. This triad is the classic presentation of a nonfunctioning pituitary macroadenoma causing mass effect. The mass compresses the infundibular stalk, severing the tonic inhibitory dopamine signal from the hypothalamus to the lactotrophs. Because lactotrophs are released from this inhibition, a mild to moderate elevation in prolactin occurs (typically <150 ng/mL). Simultaneously, the mass compresses normal anterior pituitary tissue (or disrupts hypothalamic releasing factors), causing deficits in other axes, notably central hypothyroidism and hypogonadotropic hypogonadism.
Step-by-Step Solution
Key Concept
The 'stalk effect' occurs when a nonfunctioning pituitary mass compresses the infundibular stalk, blocking the inhibitory dopamine signal from the hypothalamus and causing mild hyperprolactinemia, while simultaneously causing central hypopituitarism (e.g., central hypothyroidism) via mass effect.
Alternative Method
When evaluating an elevated prolactin level, always check the absolute magnitude: levels <150 ng/mL with a known mass suggest stalk effect (dopamine disconnect), whereas levels >200 ng/mL suggest a true prolactin-secreting tumor.
Estimated Time:1m 30s