Prolactin is the milk-production hormone that also silences the reproductive axis when elevated. A high reading explains unexplained milk discharge, irregular cycles and low libido, and common medicines are the most frequent culprit.
Standard range versus optimal range
| Framework | Prolactin value | Source |
|---|---|---|
| Women, typical reference | below 25 ng/mL | Endocrine Society 2011 |
| Men, typical reference | below 20 ng/mL | Endocrine Society 2011 |
| Prolactinoma territory | often above 100-200 ng/mL | Endocrine Society 2011 |
When it runs high
High prolactin suppresses GnRH and with it the whole reproductive axis, producing irregular or absent periods, milk discharge, low estrogen or testosterone and infertility. Levels above about 200 ng/mL almost always mean a prolactin-secreting pituitary adenoma.
- Prolactinoma, the most common functioning pituitary tumor
- Antipsychotics, metoclopramide and some antidepressants
- Pregnancy and breastfeeding, physiological
- Hypothyroidism, through excess TRH
- Chest wall stimulation and nipple piercings
- Chronic kidney disease reducing clearance
- Macroprolactin, an inactive antibody-bound form that inflates some assays
When it runs low
Low prolactin rarely causes clinical problems and is usually an incidental finding. Very low values after pituitary damage can impair milk production after childbirth, which matters only in that setting.
- Pituitary failure, rare
- Dopamine agonist medication
- Normal variation, the usual case
One hormone, many impostors
Before blaming the pituitary, every high prolactin deserves three checks. Medications top the list, especially antipsychotics and the gut drug metoclopramide. Second is macroprolactin, a harmless antibody-bound form that many assays count as active hormone, easily excluded by a precipitation step the lab can run on request. Third is the hook effect, where enormous values from a large tumor paradoxically read low, which is why imaging follows any convincing clinical picture even with modest numbers.
How it silences reproduction
Prolactin whispers to the hypothalamus to slow GnRH pulses, and the ovaries or testes go quiet downstream. This is the mechanism that spaces pregnancies during breastfeeding and the same one that causes infertility, low libido and bone loss when prolactin rises for other reasons. Men present late, often with headaches and vision changes from a tumor already large, because low libido is easy to dismiss.
Treatment is usually tablet-simple
Dopamine agonists such as cabergoline shrink prolactinomas and normalize levels in the great majority, making this one of the few brain tumors treated primarily with pills. Drug-induced cases reverse when the culprit is swapped. Mild elevations under about 50 ng/mL with normal cycles and no symptoms often just need a stress-free morning repeat, since venipuncture anxiety alone can bump the number.
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