Prolactin: When a Hormone Disrupts Your Periods

Prolactin is best known as the hormone that enables breastfeeding, but it plays a quieter role in the background at all times. When its level rises higher than expected outside pregnancy and breastfeeding, it can interfere with the menstrual cycle and fertility. Because the symptoms can be subtle, prolactin is easy to overlook — but it is a simple hormone to measure.

What prolactin does

Prolactin is produced by the pituitary gland, a small structure at the base of the brain. Its main job is to stimulate milk production after childbirth. To make room for this, high prolactin naturally suppresses the reproductive hormones that drive ovulation — which is why breastfeeding often pauses periods. The problem arises when prolactin is raised at other times, producing the same suppressive effect when it is not wanted.

How it affects periods and fertility

When prolactin is high, ovulation can become irregular or stop, leading to a range of symptoms:

• Irregular, infrequent or absent periods
• Difficulty conceiving
• Milky nipple discharge unrelated to breastfeeding
• Reduced libido
• Occasionally headaches or changes in vision, if a pituitary cause is large

Why prolactin rises

There are several possible reasons, and not all are worrying. Common causes include stress, recent exercise, nipple stimulation, and certain medications — including some antidepressants and anti-sickness drugs. An underactive thyroid can also raise prolactin. Less commonly, a benign growth on the pituitary gland called a prolactinoma is responsible. Because everyday factors can nudge the level up, a single high result is often repeated before drawing conclusions.

Getting an accurate result

Prolactin can spike briefly with stress, a recent meal or physical activity, so results are most reliable when the sample is taken in a calm, rested state, ideally later in the morning. If prolactin is investigated as part of a wider look at irregular periods, cycle-timing considerations apply to the other hormones — reproductive hormones such as FSH and oestradiol are usually checked early in the cycle on day 2 to 5, and a day-21 progesterone test can confirm whether ovulation is occurring.

When to get checked

Consider a prolactin test if your periods have become irregular or stopped without pregnancy, you are having difficulty conceiving, or you notice unexpected milky discharge. Seek prompt medical advice if you have persistent headaches or visual changes alongside these symptoms. A raised result should always be interpreted by a clinician, who can decide whether repeat testing, thyroid checks or imaging are needed.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic