A prolactinoma is a benign tumour of the pituitary gland that produces an excess amount of the hormone prolactin. Prolactin is a hormone that is secreted from the pituitary gland. It has many different functions in the body including controlling milk production and helping to regulate the function of the reproductive system. The pituitary gland is the ‘master gland’ which produces several hormones that control the function of a number of other glands around the body. It is situated in a bony hollow at the base of the skull under the middle of the brain.
According to its size, a prolactinoma can be either a microprolactinoma (smaller than 1 cm in diameter) or a macroprolactinoma (larger than 1 cm in diameter).
The pituitary gland contains several types of cells each of which are responsible for the production of a different hormone. A prolactinoma occurs when the cells that are responsible for the production of the hormone prolactin (known as lactotroph cells) start to multiply for no apparent reason. This can lead to the development of a growth of variable size within the pituitary gland. The majority of prolactinomas are less than 1 cm in size. The increase in number of prolactin-producing cells leads to excess production of the hormone prolactin, leading to high levels in the blood.
Symptoms of a prolactinoma relate to the high prolactin levels in the blood. Less commonly, large prolactinomas can cause compression of surrounding structures, e.g. visual nerves.
Symptoms from the high prolactin levels include discharge of milk from the breast (galactorrhoea) and breast tenderness. These symptoms are more common in women than men. High prolactin levels can also affect the function of the ovaries or testes by interfering with the hormones that control these glands. In women, this can lead to irregularity (oligomenorrhoea) or even a complete stopping of menstrual periods (amenorrhoea), reduced fertility and menopausal symptoms, like hot flushes. In men, excess prolactin can cause loss of (or decreased) sex drive, low energy levels, difficulty obtaining an erection and reduced fertility. If left untreated, this interference with the function of ovaries or testes can also lead to osteoporosis.
Most prolactinomas are small (microprolactinomas), so the tumour mass itself does not usually cause specific symptoms. Large prolactinomas (macroprolactinomas) can cause compression of the normal pituitary gland. This can affect the release of the other pituitary hormones (see the article on hypopituitarism). They may also be associated with visual impairment due to compression of the nerves to the eyes.
Prolactinomas are the most common form of pituitary gland tumours associated with excess hormone production. About 1 in 10,000 people have a prolactinoma. It can occur in both sexes and at any age, but it is more common in women aged 20–50 years. Microprolactinomas are much more common than macroprolactinomas.
There is no evidence of inheritance in the majority of cases of prolactinoma. Rarely, prolactinomas can occur as part of a condition called multiple endocrine neoplasia type 1, which can be inherited. Recently, several other genes – AIP (aryl hydrocarbon interacting protein) and SDH (succinate dehydrogenase) have been described in patients with prolactinomas, and rare prolactinoma families suggest the presence of an as yet unknown gene or genes in these individuals.
If the doctor suspects that a patient’s symptoms are related to a high prolactin level, the first test is a blood test to check the prolactin level (larger tumours tend to cause higher prolactin levels). If levels are high, a repeat blood test is taken to recheck the prolactin level. A test for levels of other hormones produced from the pituitary gland will also need to be carried out. These tests can all be performed as an outpatient.
The doctor might also request an eye test to assess the field of vision if it is suspected that the prolactinoma is pressing on the nerves to the eyes.
A prolactinoma requires treatment only if it is causing symptoms related to high levels of prolactin in the blood level or because of its size. The aim of any treatment is to normalise blood prolactin levels and reduce the size of large tumours.
Most prolactinomas respond well to treatment with medication in tablet form. This treatment is very effective in both normalising prolactin levels in the blood and reducing the size of prolactinomas. Occasionally, surgery or radiation therapy is necessary if a prolactinoma does not respond to medication or if the patient cannot tolerate the medication.
Medication used to treat prolactinomas belongs to a group called ‘dopamine agonists’. There are several different types of this medication that can be prescribed and the doctor will tailor the medication to suit the patient’s needs. The most commonly used is cabergoline because of its effectiveness and the fact that most patients can cope with it without too many ill-effects. It is taken once to twice per week. Similar drugs include quinagolide or bromocriptine. Bromocriptine is taken two or three times daily. Quinagolide is taken once per day.
Most patients will be carefully monitored when they start taking the medication to ensure that the dose they are given is correct. Side-effects of dopamine agonists can include nausea, vomiting, loss of appetite, dizziness and drowsiness. Side-effects are particularly noticeable with bromocriptine, which needs to be increased in dose gradually.
Some patients with Parkinson’s disease taking large doses of cabergoline develop heart valve thickening and this has led to some discussion as to whether patients with prolactinomas should have regular ‘screening’ ultrasound scans of the heart (echocardiogram). It is important to note that the doses of cabergoline used in Parkinson’s disease are much higher than those used to treat prolactinomas (typically 3 g per day rather than 0.5–1 mg per week). Together with a number of reassuring studies that have recently been published on this issue, most endocrinologists do not routinely arrange regular echocardiograms and believe cabergoline is a very safe drug at low doses.
The outcome for patients with prolactinomas is usually excellent as the majority respond well to medication. However, they will require regular monitoring by their GP and/or endocrinologist. As the prolactin level returns to normal, the function of the ovaries and testes returns, including recovery of fertility, and all related symptoms disappear. Contraception should be used in the normal way to prevent pregnancy. However, if conception is desired, the doctor will usually ask the patient to stop taking the ‘dopamine agonist’ tablet once pregnancy is confirmed. However, a small proportion of patients (generally those with larger tumours) will need to continue with dopamine agonist therapy during pregnancy. The frequency of follow-up during pregnancy will depend on the size of the prolactinoma.
In the long-term, a proportion of prolactinomas enter remission, allowing the dopamine agonist treatment to be discontinued. The doctor might stop treatment after a period of time (usually after two or more years’ treatment), followed by careful monitoring in case it reappears.
Last reviewed: Dec 2014