Prolactin

Test in the morning, 2–3 hours after waking. Avoid nipple stimulation, breast examination, stressful exercise, and heavy meals in the preceding hours. Prolactin is higher during sleep, after meals, and with stress — a single mildly elevated result is often not repeatable on a properly collected sample. Significant elevation should be confirmed on a repeat draw.

Common culprits: antipsychotics (especially risperidone and haloperidol), metoclopramide and domperidone (anti-nausea), some antidepressants (SSRIs less than tricyclics), verapamil, methyldopa, oral estrogen, opioids, and some anti-seizure drugs. Always review medication history before pursuing imaging. A trial of stopping or substituting the offending drug (with the prescriber's input) often resolves mild elevations.

Macroprolactin is a large complex of prolactin bound to antibody, biologically inactive but detected by standard prolactin assays. It causes falsely elevated prolactin results without any symptoms or pituitary lesion. If prolactin is high but symptoms are absent, the lab can perform a polyethylene glycol (PEG) precipitation test to detect macroprolactin and avoid unnecessary pituitary imaging.