Prolactin

No special preparation is needed. A blood sample is taken from a vein in your arm. 

Tell your doctor about all medicines and supplements you are taking, including over-the-counter vitamins. Some medicines and supplements — including biotin — can affect the result. 

What the test measures 

Prolactin is produced by the pituitary gland — a small gland at the base of the brain. It is controlled by dopamine, a brain chemical that normally keeps prolactin production in check. Its main role is to stimulate breast milk production. 

In people who are not pregnant or breastfeeding, prolactin levels are normally low. During pregnancy and while breastfeeding, high prolactin levels are completely normal and expected.

If your prolactin is high (hyperprolactinaemia) 

A raised prolactin — called hyperprolactinaemia — has several possible causes. Your doctor will work with you to find out which applies to you. 

Common causes include: 

• Prolactinoma — the most common cause of persistently raised prolactin. A prolactinoma is a benign (non-cancerous) tumour of the pituitary gland that produces excess prolactin. Most are small and treatable. 
• Medicines — many medicines raise prolactin, including some antipsychotics (such as risperidone and haloperidol), some antidepressants, some blood pressure medicines, some medicines for acid reflux (such as metoclopramide and domperidone), and oestrogens 
• Hypothyroidism — an underactive thyroid is a common and treatable cause of raised prolactin 
• Polycystic ovary syndrome (PCOS) — raised prolactin can be associated with PCOS 
• Kidney disease — the kidneys help clear prolactin from the body; kidney disease can cause levels to rise 
• Other pituitary or hypothalamic conditions — diseases affecting the pituitary gland or the brain region above it 
• Anorexia nervosa 
• Pregnancy and breastfeeding — It is normal for prolactin to increase 

Macroprolactin — a common reason for a raised result that does not need treatment 

Some people have a raised prolactin result simply because most of their prolactin is in a different, larger form called macroprolactin. Macroprolactin is not produced by the pituitary gland and is not active in the body. It does not cause symptoms and does not indicate any disease. 

If your prolactin result is raised, your laboratory should routinely check whether macroprolactin is the reason. If macroprolactin is found to be the cause, no further investigation or treatment is usually needed. Your doctor will explain this if it applies to you. 

If your prolactin is low 

Low prolactin levels are uncommon and are not usually treated on their own. They may indicate reduced overall pituitary function (hypopituitarism), where the pituitary gland is not producing enough hormones generally. Some medicines can also lower prolactin, including dopamine, levodopa, and certain ergot-based medicines. 

About prolactinomas 

Prolactinomas are the most common type of pituitary tumour. They are almost always benign (non-cancerous) and most are small. They occur more often in women but can affect men too. 

A prolactinoma can cause symptoms in two ways: by producing too much prolactin (causing symptoms such as galactorrhoea, irregular periods, or fertility problems), or by growing large enough to press on nearby structures — which can cause headaches and visual problems. 

Most prolactinomas respond well to medicine. The medicines used — bromocriptine and cabergoline — work by mimicking dopamine and suppressing prolactin production. Treatment can reduce prolactin levels, shrink the tumour, and restore fertility. Surgery is sometimes needed for larger tumours or those that do not respond to medicine. 

A mildly raised prolactin result on a single sample often needs to be repeated before any action is taken. Stress, the time of day, and physical activity can all temporarily raise levels. Your doctor may ask you to have a second test under calmer conditions. 

If prolactin is persistently raised, your doctor will investigate the cause. This usually includes checking for macroprolactin, testing thyroid function, reviewing your medicines, and considering an MRI scan of the pituitary gland. 

If a prolactinoma is found, your doctor will work with you on a treatment plan. Most people respond well to medication, and fertility and symptoms often improve with treatment. Regular monitoring will continue to check your response and watch for any changes. 

Prolactin is one of the tests most affected by external factors at the time of sampling: 

• stress and anxiety — including the stress of a blood test itself — can temporarily raise prolactin 
• time of day — levels are highest in the morning shortly after waking; ideally the test should be done a couple of hours after waking, after 30 minutes of rest 
• recent physical activity, nipple stimulation, or a large meal can also cause small temporary rises 
• pregnancy and breastfeeding — high prolactin in these situations is completely normal 
• medicines — many common medicines raise prolactin (see above); always give your doctor a full medicines list 
• Biotin supplements — biotin (vitamin B7), found in many over-the-counter supplements, can interfere with certain laboratory methods and cause a falsely low prolactin result. Tell your doctor if you are taking any supplements, even those not labelled as containing biotin, as some nutritional supplements contain it without prominently listing it 
• macroprolactin — can cause a falsely high result that appears significant but requires no treatment 

Prolactin is usually investigated alongside other tests. Your doctor may also request: 

• Thyroid function tests — hypothyroidism is a common and easily treatable cause of raised prolactin 
• Testosterone test — in men, raised prolactin often causes low testosterone and in women high testosterone can indicate PCOS. 
• FSH test and LH test — to assess ovulation and fertility in women 
• Oestrogen and progesterone test — further hormone tests as part of a fertility assessment 
• IGF‑1 and other pituitary hormones — if a broader pituitary assessment is needed 
• MRI scan of the brain — to look for a prolactinoma or other pituitary abnormality (not a blood test, but commonly requested alongside) 
• eye examination — to assess for visual field changes if a larger pituitary tumour is suspected