Low prolactin: what it means

Prolactin is made by the pituitary gland, a small gland at the base of the brain, and its main role is to stimulate breast milk production. The Pituitary Foundation explains that it is secreted in large amounts during pregnancy and breastfeeding but is present at all times in both men and women, in small amounts.

In people who are not pregnant or breastfeeding, Lab Tests Online UK says prolactin levels are normally low. This is one reason a result at the bottom of a reference range is common and often unremarkable, and why laboratories and clinicians spend far more time on high results.

Lab Tests Online UK says low prolactin levels are uncommon and are not usually treated on their own, but may indicate reduced overall pituitary function. The pituitary produces six main hormones, according to the Pituitary Foundation: growth hormone, LH and FSH, ACTH, TSH, vasopressin (AVP) and prolactin. When the gland is damaged, one or several can fall.

The Pituitary Foundation adds that having low prolactin alone does not always mean someone has hypopituitarism, since prolactin levels are affected by many factors, such as stress or medications. Clinicians therefore look at the other pituitary hormones and the story, rather than treating a low prolactin as a diagnosis.

When low prolactin is part of a wider pituitary problem, the Pituitary Foundation lists several causes. The most common is a benign tumour in the pituitary gland, which may press on the gland and stop it working correctly. Others include surgery or radiotherapy for a pituitary tumour, pituitary cysts, tumours around the gland, serious head injuries, reduced blood flow or bleeding into the pituitary, inflammation of the gland (hypophysitis) and genetic conditions.

• Pituitary tumours or cysts.

• Surgery or radiotherapy in the pituitary area.

• Serious head injury.

• Loss of blood flow to the gland, including Sheehan's syndrome after severe childbirth bleeding.

• Inflammation of the pituitary (hypophysitis), most commonly seen during pregnancy.

• Rare inherited conditions.

One situation in which low prolactin has particular meaning is after a birth with severe bleeding. The Pituitary Foundation describes Sheehan's syndrome as postpartum hypopituitarism that may occur after severe uterine haemorrhage or very low blood pressure during childbirth. Prolactin is among the hormones that can be deficient.

It lists signs that may include difficulty breastfeeding or a reduced milk supply, diminishing body hair, weight gain, absent or irregular periods, dizziness and fatigue. There may be very few symptoms and they may not occur immediately after giving birth. Diagnosis relies on the history of the birth and blood tests, and an MRI or CT scan of the pituitary may be performed. Treatment replaces whichever hormones are deficient.

Many mothers have difficulty breastfeeding for reasons unrelated to the pituitary, so this explanation applies only when other features fit, particularly heavy bleeding at delivery.

Lab Tests Online UK says some medicines can lower prolactin, including dopamine, levodopa and certain ergot-based medicines. It also says biotin (vitamin B7), found in many over-the-counter supplements, can interfere with certain laboratory methods and cause a falsely low prolactin result, and that some supplements contain biotin without listing it prominently.

Time of day and stress mainly affect results in the other direction, making them higher, but these factors illustrate why a single unexpected result is usually repeated. If your result was low and you take biotin, ask whether the test should be repeated after stopping it for the period the laboratory advises.

If prolactin is low and you feel well, a clinician may do no more than note it. If you have symptoms of pituitary underactivity, such as persistent fatigue, missing periods, low libido, dizziness or a history of significant blood loss at delivery, the Pituitary Foundation says blood tests are used to measure the hormone levels, sometimes first thing in the morning because the levels of many pituitary hormones are highest then. Some hormones need a stimulation test.

There is no routine treatment to raise prolactin itself. When hypopituitarism is confirmed, treatment usually means replacing the hormones that are missing, such as levothyroxine, hydrocortisone, oestrogen or testosterone, depending on which are deficient. Care is usually led by an endocrinologist.