A 35-year-old man with a pituitary macroadenoma has a serum prolactin of 3800 ng/mL. He undergoes venous sampling from both inferior petrosal sinuses and peripheral blood during CRH stimulation. The central:peripheral prolactin gradient is 2.0. What is the significance of this finding?
- A Confirms a prolactinoma; gradient >2 localizes tumor to the pituitary
- B Indeterminate; the gradient threshold of 2.0 is used for ACTH, not prolactin ✓
- C Suggests ectopic prolactin secretion since the gradient should exceed 3.0
- D Confirms stalk compression hyperprolactinemia; dopamine gradient is the key measure
Explanation
The inferior petrosal sinus sampling (IPSS) with CRH stimulation uses a central:peripheral ACTH gradient of ≥2 basally and ≥3 post-CRH to distinguish pituitary Cushing's disease from ectopic ACTH. For prolactin, the clinical value of the central:peripheral gradient in IPSS is not an established diagnostic criterion. Prolactin-secreting adenomas are diagnosed by markedly elevated serum prolactin (>200 ng/mL strongly suggests prolactinoma); very high levels like 3800 ng/mL are virtually diagnostic without IPSS. Options A and C misapply prolactin-specific gradients that are not standard; option D incorrectly frames this as a dopamine measurement.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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