Laboratory Evaluation for Nipple Discharge

Characterization and Laboratory Testing

The American College of Radiology recommends characterizing nipple discharge as physiologic (bilateral, multiple ducts, white/green/yellow/clear color) or pathologic (unilateral, single duct, spontaneous, bloody/serous) to determine the need for laboratory testing 2, 3
For physiologic discharge suspected of being due to hyperprolactinemia, the initial laboratory test should be serum prolactin level, which can be drawn at any time of day and does not require fasting, using age-specific and sex-specific reference ranges 1
The American College of Radiology suggests that primary hypothyroidism, which causes hyperprolactinemia in 40-43% of frank hypothyroidism cases, should be evaluated with thyroid-stimulating hormone (TSH) tests 1
Renal function tests (creatinine, BUN) and liver function tests should be conducted as chronic kidney disease and severe liver disease can increase prolactin secretion and reduce renal clearance 1
Checking for macroprolactin is necessary when prolactin is mildly or incidentally elevated, as macroprolactin accounts for 10-40% of hyperprolactinemia cases and is biologically inactive 1

The American College of Radiology recommends proceeding directly to imaging, such as diagnostic mammography/digital breast tomosynthesis (DBT) and ultrasound, for pathologic discharge, without routine laboratory testing 4, 5, 2
For patients with pathologic discharge, age-appropriate imaging (mammography/DBT + ultrasound for age ≥40 years or ≥30 years; ultrasound alone for <30 years) should be conducted 4, 5, 2

A prolactin level >4,000 mU/L (188 μg/L) suggests prolactinoma and warrants pituitary MRI 1
A large pituitary mass with paradoxically normal/mildly elevated prolactin requires serial dilutions to evaluate for "hook effect" 1
Before attributing hyperprolactinemia to prolactinoma, a thorough medication review should be conducted, as dopamine antagonists are among the most common causes of hyperprolactinemia 1