Prolactin is a hormone produced by the pituitary gland, a small pea-sized gland located in the base of the brain. The name “prolactin” refers to the role of prolactin in lactation (breast milk production). In women, prolactin levels in the blood rise during pregnancy and childbirth to prepare the breasts for upcoming breastfeeding and remain high as long as the child continues to breastfeed.

But the role of prolactin is not limited to lactation. It is actually one of the most versatile hormones in the body. It is produced in both men and women and regulates a wide range of processes such as the immune system, mental health, metabolism and stress responses.

In short, it is useful to analyse prolactin levels in the blood to:

• Investigate the causes of infertility or irregular periods in women

• Investigate causes of infertility or erectile dysfunction in men

• Detect prolactin-producing tumours (prolactinomas)

• Detect hypopituitarism

In women, elevated prolactin levels in the blood can cause irregular menstrual cycles, problems with ovulation, infertility and abnormal breast milk production. In men, elevated prolactin levels lead to low testosterone, enlarged breasts, erectile dysfunction and also fertility problems. It is therefore common to measure prolactin levels in both male and female infertility workups.

The most common cause of increased prolactin levels is the presence of a small prolactin-producing tumour in the pituitary gland called a prolactinoma. The tumour is usually benign, which means that it is not cancerous. But if it is big enough, it can cause symptoms such as headache and problems with vision due to compression of the visual nerves. Prolactinomas are more common in women than men.

Low levels of prolactin are most often not a problem, but can appear together with low levels of other pituitary hormones such as growth hormone and thyroid stimulating hormone (TSH).

The reference range for prolactin levels in the blood can be different depending on the laboratory and technique used. Doctors usually also take into account a number of factors when evaluating albumin values.

High prolactin levels in the blood may be associated with:

• Underactive thyroid (hypothyroidism)

• Polycystic ovary syndrome (PCOS)

• Prolactinoma

• Other pituitary diseases

• Anorexia nervosa

• Pregnancy

• Breastfeeding

• High level of stress

Low prolactin levels in the blood may be associated with:

• Hypopituitarism

The level of prolactin varys during the day and is the highest during the morning in people with normal circadian rhythm. The prolactin tests should therefore be taken before 10 am in the morning for correct interpretation.

Macroprolactin, an inactivated form of prolactin that is present in a small number of people, can be detected by prolactin tests. Presence of macroprolactin can give a falsely high level of prolactin. High levels of macroprolactin are not associated with any symptoms or disease. The presence of macroprolactin can be suspected in people who have high prolactin levels but no symptoms of any disease.

Ursula B. Kaiser, MD. Hyperprolactinemia and infertility: new insights. J Clin Invest. 2012 Oct 1; 122(10): 3467–3468.