What is the interpretation of a prolactin level measured on the second day of the menstrual cycle?

From the Guidelines

A prolactin level measured on the second day of the menstrual cycle is considered a valid measurement for assessing prolactin levels, as prolactin remains relatively stable throughout the menstrual cycle. Prolactin levels typically fluctuate only minorly during the menstrual cycle, making the second day an acceptable time for measurement 1. Normal prolactin values generally range from 5-25 ng/mL (or 5-25 μg/L) for non-pregnant women, though reference ranges may vary slightly between laboratories.

Key Considerations

• If the level is elevated above the reference range, it may indicate conditions such as prolactinoma, medication effects, hypothyroidism, or other pituitary disorders.
• Stress, sleep disruption, breast stimulation, and certain medications can transiently increase prolactin levels, so these factors should be considered when interpreting results.
• If an elevated level is found, repeat testing is often recommended, preferably in the morning after fasting and avoiding breast stimulation, to confirm the elevation and rule out transient causes.
• The timing on day 2 of the cycle is advantageous as it avoids the mid-cycle surge of other hormones that might indirectly affect prolactin, providing a relatively stable baseline for assessment.

Additional Recommendations

• Assess baseline macroprolactin levels where serum prolactin is found to be mildly or incidentally elevated, as macroprolactinemia can be present in individuals with hyperprolactinemia 1.
• Consider the potential for macroprolactinemia, which has low biological activity, and may not require treatment, highlighting the importance of secondary analysis to detect its presence.

From the Research

Prolactin Levels During the Menstrual Cycle

• Prolactin levels can vary throughout the menstrual cycle, with studies indicating changes in levels during different phases 2, 3, 4, 5.
• One study found that the mean prolactin value changed in each phase of the menstrual cycle, with the highest value recorded in the periovulatory period 2.
• Another study reported a progressive and significant increase in serum prolactin during the late follicular phase, with a maximal value concomitant to the LH peak 4.
• Prolactin levels were also found to be significantly higher during the luteal phase than during the early follicular phase 4.

Interpretation of Prolactin Levels on the Second Day of the Menstrual Cycle

• The second day of the menstrual cycle typically falls within the follicular phase.
• According to one study, prolactin levels during the follicular phase are generally lower compared to the ovulatory and luteal phases 2, 5.
• However, it is essential to consider that prolactin levels can vary significantly between individuals and throughout the menstrual cycle 5.
• To accurately interpret prolactin levels, it is crucial to consider the phase of the menstrual cycle and apply specific reference intervals for each phase 5.

Reference Intervals for Prolactin Levels

• One study suggested that a single reference interval could be used if prolactin is only measured in the follicular phase, well before midcycle 5.
• However, further studies are needed to confirm formal reference ranges for different phases of the menstrual cycle and populations, such as postmenopausal women and men 5.