High prolactin: causes, symptoms and what happens next

Prolactin is a hormone produced by the pituitary gland, a small gland at the base of the brain. Lab Tests Online UK explains that it is controlled by dopamine, a brain chemical that normally keeps prolactin production in check, and that its main job is to stimulate breast milk production. In people who are not pregnant or breastfeeding, levels are normally low, and during pregnancy and breastfeeding high levels are completely normal and expected.

A raised result on its own is not a diagnosis. Patient.info describes hyperprolactinaemia as a relatively common problem, and a UK study it cites found a prevalence of 0.23 percent, with the cause being a pituitary disorder in about 46 percent and drug-induced in about 46 percent, among other causes. The rate was 3.5 times higher in women than in men, and highest in women aged 25 to 44.

The key point for anyone who has been told their prolactin is high is that the cause needs to be worked out step by step, and many of the possible causes are straightforward to deal with.

The Pituitary Foundation lists common causes as pregnancy, nipple stimulation and suckling, stress, certain medicines, an underactive thyroid gland and a benign finding called macroprolactinaemia. Once these have been excluded, doctors consider a prolactinoma.

Patient.info gives a fuller list and groups the causes. Physiological causes include pregnancy, the period after childbirth, breast stimulation and physical or emotional stress, including the stress of venepuncture. Endocrine and metabolic causes include hypothyroidism, Cushing's syndrome, liver cirrhosis and polycystic ovary syndrome. Intracranial causes include prolactin-secreting pituitary tumours, non-secreting pituitary tumours that interfere with dopamine reaching the prolactin-producing cells, and damage from head injury, brain surgery or radiotherapy. Chest wall surgery or trauma and sarcoidosis are listed among other causes. Lab Tests Online UK adds kidney disease and anorexia nervosa.

• Medicines: anti-sickness medicines such as metoclopramide and domperidone, acid-reducing medicines such as omeprazole, some antidepressants and antipsychotics such as risperidone, some blood pressure medicines and oestrogens.

• An underactive thyroid, which the Pituitary Foundation says can be diagnosed by a simple blood test.

• A prolactinoma, described by Lab Tests Online UK as the most common cause of persistently raised prolactin.

• Pregnancy and breastfeeding, when raised levels are normal.

• Macroprolactin, a large inactive form that makes the number look high without disease.

Many people have no symptoms at all. When symptoms do occur, they depend on sex and on the cause. Lab Tests Online UK lists, in women, milk leaking from the breasts when not pregnant or breastfeeding (galactorrhoea), irregular or absent periods, difficulty getting pregnant, and unexplained headaches or visual disturbances. In men it lists reduced sex drive or erectile dysfunction, rarely milk leaking from the breasts, low testosterone, infertility and headaches or visual disturbances.

The Pituitary Foundation says women with a prolactinoma may first notice loss of periods, low libido, vaginal dryness and infertility, and that galactorrhoea is not a sign of breast disease and does not raise the risk of breast cancer. It says men more often have larger tumours, with low testosterone, low libido, impotence and a low sperm count. It also advises that squeezing or expressing milk acts as stimulation and keeps prolactin raised.

A larger tumour can press on the nerves to the eyes, causing headache or visual problems. Patient.info lists headache, visual disturbance and, rarely, symptoms of low pituitary hormone levels. Sudden severe headache or a rapid change in vision should be treated as urgent: contact NHS 111 or go to A&E.

Patient.info says initial investigations include thyroid function tests, excluding pregnancy and a basal serum prolactin level. If the result is only mildly raised, it should be repeated before referral, and measurement on three separate occasions at least two hours after rising and when rested is sufficient. It says a level above 5000 mU/L usually indicates a true prolactinoma.

Further investigations include visual field testing and pituitary imaging, preferably MRI, plus assessment of other pituitary hormones. The Pituitary Foundation says a GP may do the first prolactin and thyroid tests and then arrange attendance at a specialist endocrine clinic for further tests and scans.

Two laboratory quirks are worth knowing. Lab Tests Online UK says the laboratory should routinely check whether macroprolactin explains a raised result, and that some medicines and supplements, including biotin, can affect the result. Patient.info adds that, rarely, a very large tumour can be accompanied by a falsely low prolactin result because of a laboratory effect, which is why laboratories may dilute the sample.

Treatment depends on the cause. If a medicine is responsible, a doctor may review it, and you should not stop a prescribed medicine yourself. If hypothyroidism is the cause, treating the thyroid is the answer. Patient.info states that asymptomatic people with raised prolactin or a small prolactinoma do not necessarily need treatment, noting that 90 to 95 percent of prolactinomas never increase in size.

When treatment is needed, the Pituitary Foundation says it is likely to be tablets, with dopamine agonists as first-line medication. The three it names are cabergoline, bromocriptine and quinagolide. Prolactin levels often fall to normal within a few weeks; in women periods usually resume and fertility is restored in most cases, in men testosterone may rise, and nearly all prolactinomas shrink. Side effects include dizziness on standing, nausea, headache, tiredness and constipation, and both sources warn that impulse control problems such as pathological gambling can occur, so family members should know what to look out for.

If tablets do not work, Patient.info says surgery may be used to reduce the tumour, while radiotherapy is rarely used. Pregnancy planning matters: Patient.info notes a small risk of tumour enlargement in pregnancy, particularly with macroadenomas, and that people should be under an endocrinologist.

A raised prolactin is usually first picked up through a GP blood test. Repeat testing, thyroid tests and referral to an NHS endocrinology clinic for scans and long-term treatment follow from there. Private endocrinology consultations and MRI are also available in the UK; the investigations and treatments are broadly the same.