Background:  Prolactin is not commonly recognised as a hormone that changes significantly within the menstrual cycle or after menopause. The aim of this study was to determine the degree of variability of prolactin in these physiological states.
Method:  Prolactin levels obtained from 6540 subjects between January 2006 and November 2008 were divided into five groups: men, postmenopausal women and premenopausal women in follicular/non‐cycling, ovulatory and luteal phases. The median and 97.5th centile was determined for each group. The 97.5th centile was used to define the upper limit of prolactin.
Results:  The prolactin median and upper limits were not significantly different in men and postmenopausal women. They were significantly higher in premenopausal women compared to men and postmenopausal women. Within premenopausal women, the prolactin median and upper limits were significantly higher in ovulatory phase compared to follicular/non‐cycling and luteal phases and in luteal phase compared to follicular/non‐cycling phase.
Conclusions:  Prolactin levels varied significantly throughout the menstrual cycle, and the utility and accuracy of prolactin testing may be improved by applying specific reference intervals for each phase of the menstrual cycle. Alternatively, a single reference interval could be used if prolactin is only measured in the follicular phase, well before midcycle. Prolactin levels in postmenopausal women and men were not significantly different, and a common prolactin reference interval may be appropriate. Further studies to confirm formal reference ranges for these groups may be clinically helpful.