Prolactin is one of the most clinically significant hormones in reproductive medicine, and it exists in a blind spot. Most people have never heard of it. Most GPs test it only after other investigations come back normal. And the symptoms it produces (absent periods, low libido, unexplained infertility, galactorrhoea) are frequently attributed to stress, contraception, or "just one of those things."
Despite being best known for its role in breastfeeding, prolactin has over 300 identified biological actions. When it's elevated outside of pregnancy or breastfeeding, the downstream effects can be significant. This article covers what prolactin does, what happens when levels are off, and when testing makes sense.
A note before we get into it
General information only. I'm not an endocrinologist. Elevated prolactin can have multiple causes, some of which require specialist investigation (including pituitary imaging). If your prolactin is elevated, work with your GP or an endocrinologist to determine the cause and appropriate management.
What prolactin does
Prolactin is produced by the anterior pituitary gland, a pea-sized structure at the base of the brain. Its name literally means "for lactation," and yes, its most famous role is stimulating breast milk production after childbirth.
But outside of pregnancy and breastfeeding, prolactin circulates at low levels and plays a supporting role in immune regulation, metabolic function, reproductive signalling, and behavioural responses.
The problem arises when prolactin is elevated outside of pregnancy or breastfeeding, a condition called hyperprolactinaemia. Excess prolactin interferes with the pituitary's release of GnRH (gonadotrophin-releasing hormone), which in turn suppresses FSH and LH. These are the hormones that drive ovulation in women and testosterone production in men.
The downstream effect is reproductive shutdown. In women, this means amenorrhoea, anovulation, and infertility. In men, it means low testosterone, low libido, erectile dysfunction, and in some cases, infertility.
What elevated prolactin looks like
In women
Absent or irregular periods (amenorrhoea or oligomenorrhoea). Infertility from anovulation. Galactorrhoea, which is unexpected breast milk production outside of pregnancy or breastfeeding (present in some but not all cases). Low libido. Vaginal dryness. Bone density loss from chronic oestrogen suppression.
In men
Low libido. Erectile dysfunction. Gynaecomastia (breast tissue enlargement) in some cases. Infertility from reduced sperm production. Low energy and mood changes. In cases of large prolactinomas: headaches and visual field disturbances from the tumour pressing on the optic chiasm.
The overlap problem. Every one of these symptoms has a long differential diagnosis. Absent periods can be PCOS, stress, low body weight, thyroid, or premature ovarian insufficiency. Low libido in men can be testosterone, stress, depression, or medications. Prolactin is rarely the first thing anyone considers, which is exactly why it gets missed.
The most common causes of high prolactin
Medications
This is the most common cause of elevated prolactin, and the most frequently missed.
Antipsychotics (risperidone, haloperidol, olanzapine, quetiapine) block dopamine receptors. Dopamine is the primary inhibitor of prolactin release, so blocking it allows prolactin to rise, sometimes dramatically. Virtually all typical antipsychotics and many atypical antipsychotics cause prolactin elevation.
Antidepressants. SSRIs (sertraline, fluoxetine, escitalopram) can mildly elevate prolactin. The mechanism is indirect (serotonin stimulates prolactin release), and the elevation is usually modest, but it can be clinically significant in some individuals.
Anti-nausea medications (metoclopramide, domperidone) are dopamine antagonists used for nausea and gastroparesis. They can cause significant prolactin elevation.
Opioids. Chronic opioid use can elevate prolactin.
If you're on any of these medications and experiencing symptoms of elevated prolactin, the medication should be considered as the cause before more invasive investigation is pursued. Your prescribing doctor can assess whether a medication change is appropriate.
Prolactinoma
A prolactinoma is a benign (non-cancerous) tumour of the prolactin-producing cells in the pituitary gland. It's the most common type of pituitary tumour.
Microprolactinomas (under 10mm) are far more common than macroprolactinomas. They're generally slow-growing and respond well to medication (dopamine agonists like cabergoline or bromocriptine). Most shrink significantly with treatment.
Macroprolactinomas (10mm or larger) can cause local pressure effects including headaches and visual field deficits (classically bitemporal hemianopia, or loss of peripheral vision on both sides). These require more urgent management.
The reassurance: prolactinomas are almost always benign. They're treatable. Most people with microprolactinomas live completely normal lives on medication.
Hypothyroidism
Primary hypothyroidism (elevated TSH) stimulates TRH (thyrotrophin-releasing hormone), which in turn stimulates prolactin release. This means untreated hypothyroidism can cause elevated prolactin, and the fix is treating the thyroid, not the prolactin.
This is why thyroid function should always be checked alongside prolactin. If TSH is elevated and prolactin is elevated, treat the thyroid first and recheck prolactin. It often normalises without further intervention.
Connection: Thyroid Function Tests
PCOS
Mildly elevated prolactin is found in approximately 15-20% of women with PCOS. The mechanism isn't fully understood but may involve altered dopamine signalling. The elevation is usually modest and doesn't typically require specific treatment, but it should be checked to distinguish from other causes.
Connection: PCOS Blood Tests
Stress and other causes
Physiological stress (including the stress of a blood draw), chest wall irritation, breast stimulation, exercise, and high-protein meals can all transiently elevate prolactin. This is why testing conditions matter, and why a single mildly elevated result should be confirmed with a repeat test under optimal conditions before further investigation.
Macroprolactinaemia. Some people have elevated "total" prolactin because a large proportion of their prolactin is in a biologically inactive "big" form (macroprolactin). This is a lab finding, not a disease, and it doesn't cause symptoms. If prolactin is elevated but symptoms are absent, your lab can test for macroprolactin to rule this out.
Prolactin in men
This section matters because male prolactin disorders are significantly underdiagnosed.
When men present with low libido, erectile dysfunction, fatigue, and mood changes, the investigation typically starts (and sometimes ends) with testosterone. If testosterone is low, the question becomes: why? Is it primary (testicular) or secondary (pituitary)?
Elevated prolactin is one of the causes of secondary hypogonadism, where low testosterone is caused by suppression of LH at the pituitary level. If prolactin isn't checked, the cause of low testosterone may be missed entirely.
Men with prolactinomas tend to present later than women because they don't have the amenorrhoea signal that prompts investigation. By the time they're diagnosed, prolactinomas in men are more often macroprolactinomas, with potential visual and neurological complications.
Any man with unexplained low testosterone, low libido, or erectile dysfunction should have prolactin checked as part of the hormonal investigation. It's a simple addition to the blood panel and it catches a treatable cause that would otherwise be missed.
Connection: Testosterone Testing (Men)
How the test works
Standard blood test. Serum prolactin from a venous blood draw.
When to test. Morning, between 9am and 11am. Prolactin has a mild circadian variation (higher at night, lower in the morning) and is affected by stress, so a calm morning draw produces the most reliable result.
Fasting. Not strictly required, but fasting is reasonable if you're also testing other markers.
Avoid sexual activity and breast stimulation for 24 hours before testing.
Avoid vigorous exercise on the morning of the test.
Try to be relaxed during the blood draw. Stress and anxiety can acutely elevate prolactin.
Mention all medications to your clinician, particularly antipsychotics, antidepressants, and anti-nausea drugs.
A single mildly elevated prolactin should be confirmed with a repeat test before further investigation. Transient elevations from stress, food, or testing conditions are common.
Understanding your result
Normal range (approximate, varies by lab)
Women (non-pregnant): 50-500 mU/L (or roughly 2-25 µg/L)
Men: 50-400 mU/L (or roughly 2-20 µg/L)
Pregnancy/breastfeeding: Significantly elevated (physiological).
Interpreting elevated results
Mild elevation (500-1,000 mU/L): Could be medications, stress, hypothyroidism, PCOS, macroprolactinaemia, or a very small prolactinoma. Repeat testing and investigation of causes warranted.
Moderate elevation (1,000-5,000 mU/L): More likely to indicate a microprolactinoma or significant medication effect. Pituitary imaging (MRI) usually recommended.
Significant elevation (above 5,000 mU/L): Strongly suggestive of a macroprolactinoma. Urgent specialist referral and pituitary MRI.
The "hook effect". Very large prolactinomas can produce prolactin levels so high that they paradoxically give a falsely normal or mildly elevated result on standard assays (due to antibody saturation). If a large pituitary mass is found but prolactin appears only mildly elevated, the lab should perform serial dilutions to check for the hook effect.
Tests to consider through Bloody Good
The prolactin test
Morning draw under relaxed conditions for the most accurate result.
Test it with Bloody Good:
Product: Prolactin Blood Test
Essential alongside prolactin
Thyroid function, reproductive hormones, and testosterone help identify the cause and downstream effects of elevated prolactin.
Test it with Bloody Good:
Product: Thyroid Function Test (TFT) (hypothyroidism causes prolactin elevation)
Product: Testosterone Free/Total + SHBG (particularly for men)
Product: FSH Blood Test (reproductive axis assessment)
Product: LH Blood Test (pituitary function)
Broader hormonal context
Additional hormonal markers to round out the picture.
Test it with Bloody Good:
Product: Oestradiol Blood Test (oestrogen status in women)
Product: Progesterone Blood Test (ovulation confirmation, see progesterone article)
Product: Cortisol Blood Test (stress axis context)
Comprehensive coverage. The Bloody Good Test covers 100 biomarkers including thyroid and baseline hormonal markers. Pairing with a standalone prolactin test and targeted reproductive hormones (FSH, LH, testosterone or oestradiol) provides thorough investigation.
What to do after testing
If prolactin is normal: Prolactin-related causes are ruled out. Investigation of your symptoms continues through other pathways (thyroid, iron, other hormonal markers, clinical assessment).
If prolactin is mildly elevated (single result): Repeat the test under optimal conditions (morning, relaxed, fasted). Confirm the elevation is real and not transient.
If prolactin is confirmed elevated: Check thyroid function first. If TSH is elevated, treat the hypothyroidism. Prolactin may normalise. Review medications. If you're on an antipsychotic, SSRI, or other prolactin-elevating medication, discuss with your prescriber. Request macroprolactin testing if symptoms are absent, to rule out macroprolactinaemia. If no medication cause and no thyroid cause, your GP will likely refer for pituitary MRI.
If a prolactinoma is found: Treatment is typically medical, not surgical. Dopamine agonists (cabergoline is first-line) normalise prolactin in most patients and shrink the tumour in the majority. Surgery is reserved for tumours that don't respond to medication or cause compressive symptoms.
If you're trying to conceive and prolactin is elevated: Treatment of hyperprolactinaemia typically restores ovulation in women and testosterone/sperm production in men. Fertility outcomes are generally excellent once prolactin is controlled.
Explore more biomarkers
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General information only. This article is not medical advice and is not a substitute for care from a qualified health professional. If you have concerning symptoms or urgent health issues, seek medical attention promptly.