A 40-year-old man with a large non-functioning pituitary macroadenoma has a serum prolactin of 65 ng/mL (normal up to 25). After complete transsphenoidal resection, prolactin normalises without any medical therapy. The preoperative mild hyperprolactinaemia was best explained by:

• A Co-secretion of prolactin by the same tumour cells
• B Ectopic production of prolactin by metastatic deposits
• C Stalk effect: compression of the hypothalamohypophyseal portal system abolishing dopaminergic inhibition ✓
• D Cross-reactivity of GH assay with prolactin

Explanation

Any mass lesion compressing the pituitary stalk interrupts dopamine delivery from the hypothalamus to lactotrophs, removing tonic inhibition and raising prolactin modestly, usually below 200 ng/mL. This 'stalk effect' resolves after decompression, as it did here. True prolactinomas typically produce much higher levels, often above 200 ng/mL in macroadenomas. Ectopic prolactin secretion is rare, and assay cross-reactivity between GH and prolactin does not occur.

Reference: Williams Textbook of Endocrinology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.