Pituitary Macroprolactinoma With Mildly Elevated Serum Prolactin: Hook Effect

A 45-year-old man presented with symptoms including severe headache, visual field defect, and body hair loss, which were linked to a pituitary macroadenoma with a misleadingly low serum prolactin level due to the hook effect. Initial tests showed a prolactin level of 100 ng/mL, but after serial dilution, the level was accurately measured at 6470 ng/mL. An MRI confirmed a macroadenoma affecting the optic nerve. Treatment with cabergoline, a dopamine agonist, led to symptom resolution, improved visual fields, and significant tumor shrinkage within one month.