Levels and units


Normal prolactin levels are usually:

Women: <440mIU/L (<25ng/mL)

Men: <350mIU/L (<20ng/mL)

These maximum levels vary slightly due to different standards. The level of prolactin also vary during the day and is highest during sleep. The schedule below shows prolactin level means calculated on the basis of ‘n’ people:

Proband Prolactin, ng/mL Prolactin, mIU/L
Women, follicular   phase (n=803) 12.1 257
Women, luteal phase   (n=699) 13.9 295
Women, mid cycle   (n=53) 17 360
Women, whole cycle   (n=1555) 13.0 276
Women, pregnant 1st   trimester (n=39) 16 339
Women, pregnant 2nd   trimester (n=52) 49 1039
Women, pregnant 3rd   trimester (n=54) 113 2396
Men, 21-30 (n=50) 9.2 195
Men, 31-40 (n=50) 7.1 151
Men, 41-50 (n=50) 7.0 148
Men, 51-60 (n=50) 6.2 131
Men, 61-70 (n=50) 6.9 146

On this website I use the unit mIU/L if nothing else is stated.

The table above comes from this source.


Two main units are used when measuring prolactin levels. These are mIU/L and ng/mL. The factor between them is 21.2.

Conversions explained:

  • ng/mL = nano-grams per mili-liter
  • mcg/L= µg/L = micro-grams per liter
  • mIU/L = mili-international-units per liter


ng/mL = mcg/L = µg/L

The conversion factor from mass to units is 21.2.

(ng/mL) x 21.2 = mIU/L

Example: 15ng/mL x 21.2 = 318mIU/L

The unit mUI/L refers to an international standard (IS) called ‘IS 84/500’.