When your periods come rarely and a blood test shows something off, it is easy to assume PCOS is the cause. But high prolactin, a condition called hyperprolactinaemia, can produce an almost identical picture: disrupted cycles, irregular ovulation, and sometimes fertility trouble. That is why a prolactin test is not an optional extra you can skip. It is a core part of confirming PCOS. This article explains how the two conditions differ, so you understand why your doctor wants to “rule out other causes” before labelling anything.

Why prolactin must be checked before confirming PCOS

PCOS is diagnosed using the Rotterdam criteria: two of three features, namely irregular periods or ovulation, signs of high androgens, and a polycystic appearance on ultrasound. But these criteria only apply after other conditions that can mimic the same symptoms have first been excluded. The 2023 international PCOS guideline lists prolactin as one of the mandatory exclusion tests, alongside TSH for the thyroid and sometimes 17-hydroxyprogesterone for adrenal conditions.

The logic is simple. If high prolactin is actually what is disrupting your periods, then the correct treatment is to address the prolactin, not to manage PCOS. Labelling someone with PCOS when the real cause is a small prolactinoma in the pituitary gland means the true problem gets missed. So before you accept a PCOS diagnosis, make sure prolactin really was checked. You can read the full picture of this process in the what is PCOS guide.

What hyperprolactinaemia is and why it mimics PCOS

Prolactin is a hormone from the pituitary gland, best known for producing breast milk after childbirth. When its level rises outside pregnancy and breastfeeding, it can suppress the hormones that control ovulation. The result is periods that become rare or vanish, and impaired fertility, exactly what many women with PCOS experience.

The subtle but important difference is this: PCOS itself rarely raises prolactin to a significant degree. So if your prolactin value is clearly high, a doctor will not simply attribute it to PCOS. Instead, they will investigate other causes. Common causes of hyperprolactinaemia include:

  • A prolactinoma, a benign (non-cancerous) growth on the pituitary gland that produces excess prolactin.
  • Certain medications, especially antipsychotics, some antidepressants, anti-nausea drugs, and blood-pressure medicines such as methyldopa.
  • An underactive thyroid (hypothyroidism), which indirectly raises prolactin, which is why TSH is always checked at the same time.
  • Pregnancy and breastfeeding, which is why a pregnancy test is usually done first.

Warning signs that point towards a prolactin problem

Certain symptoms lean more towards hyperprolactinaemia than PCOS, and they are worth noticing. The most distinctive is galactorrhoea, a milky discharge from the breast even when you are not pregnant or breastfeeding. This rarely happens in ordinary PCOS.

If prolactin is raised by a larger prolactinoma, it can press on nearby structures and cause persistent headaches or vision problems, particularly loss of side vision. The combination of absent periods, milky discharge, and headaches or disturbed vision is a pattern to raise with your doctor promptly, because it is not a typical PCOS picture. By contrast, signs of high androgens, such as excess facial hair, stubborn acne, and thinning scalp hair, lean towards PCOS. No single symptom is conclusive, but the overall pattern helps a doctor decide which direction to investigate.

A quick comparison: PCOS versus hyperprolactinaemia

The easiest way to grasp the difference is to look at where each problem begins and what stands out in each condition.

  • Where it starts. PCOS is a metabolic and hormonal condition involving insulin resistance and high androgens, while hyperprolactinaemia begins in the pituitary gland in the brain when too much prolactin is released.
  • Most distinctive symptoms. PCOS leans towards excess facial hair, stubborn acne, and sometimes weight gain; hyperprolactinaemia leans towards galactorrhoea and, with a larger growth, headaches or disturbed vision.
  • Blood reading. In PCOS, prolactin is usually normal or only mildly raised; a clearly high reading points more towards a separate prolactin cause.
  • Long-term risk. PCOS carries a type 2 diabetes and heart risk that needs monitoring over years, while the main concern with a large prolactinoma is pressure on the vision nerve, not metabolic risk.

Remember that this comparison only helps you understand the pattern, not make your own diagnosis. Many symptoms overlap, and only a blood test and a doctor’s assessment can tell them apart with certainty.

The prolactin test: how it is done and read

Prolactin is measured with an ordinary blood test. Several things affect the result, so how it is taken matters. Prolactin can rise temporarily after eating, exercise, stress, or breast stimulation, so the sample is best taken when you are calm and usually fasting, a few hours after waking.

A single mildly high reading does not necessarily mean a serious problem. If the rise is mild and you have no troubling symptoms, a doctor will typically repeat the test after a few weeks before drawing conclusions. They may also check for macroprolactin, an inactive form of prolactin that can give a falsely high reading without causing any symptoms; the lab separates it out with a special method before reporting the true level. If a high reading is confirmed and substantial, or if there are warning signs such as galactorrhoea or vision changes, then an MRI of the pituitary is considered to look for a prolactinoma.

As a rough guide, a very high reading points more strongly to a growth, whereas a moderate rise often comes from medication, the thyroid, or other milder causes. That is why a doctor reads the value alongside your symptoms and current medicines, rather than looking at a single number in isolation. Recent research also finds that mildly raised prolactin in women with PCOS usually comes from a temporary rise or macroprolactin, rather than from PCOS itself, so a proper investigation is still done.

Why the treatment is completely different

This section explains why telling the two apart is not just about labels. PCOS treatment centres on managing symptoms and long-term risk: lifestyle, medicines such as metformin for insulin resistance, hormonal pills to regulate periods, or specific medicines to stimulate ovulation when trying to conceive. No medicine “cures” PCOS; the goal is to control the condition.

Hyperprolactinaemia from a prolactinoma is treated differently. First-line treatment is usually a dopamine-agonist medicine such as cabergoline or bromocriptine, which lowers prolactin, shrinks the growth, and often restores periods and fertility. The evidence for this approach is strong and widely accepted. In other words, if high prolactin is the real cause of your disrupted periods, this medicine can fix the problem at its root, something PCOS medicines do not do. This difference is exactly why excluding prolactin early in the process matters so much; it points to a completely different treatment path. The choice and dose of these medicines are set by a specialist and should not be started on your own.

Can you have both at the same time

This question comes up often, and the answer is yes, although it is rare with a significant prolactin rise. Someone can meet PCOS criteria and at the same time have a separate cause of high prolactin, for example because they take an antipsychotic or have an underactive thyroid. In that situation, each problem may need handling separately; regulating periods alone will not fix the prolactin cause, and vice versa.

The key is that a doctor does not assume one diagnosis automatically explains everything. If your prolactin is higher than expected for ordinary PCOS, it is still investigated as its own issue. This is what “ruling out other causes” means: not to deny PCOS, but to make sure nothing more treatable is overlooked.

The Malaysian context: where to start

In Malaysia, this testing begins at a KKM Klinik Kesihatan. For citizens, one visit costs around RM1 and usually includes basic investigations. A medical officer can arrange hormone blood tests including prolactin and TSH, and if needed, refer you to a hospital O&G or endocrine clinic for further assessment, with specialist follow-up still low-cost in the public system. Private clinics and hospitals offer the same tests faster but at varying cost, so it is best to ask first.

For women who are unmarried or have never had intercourse, remember that an abdominal ultrasound (TAS) is the usual first choice in Malaysia, not a vaginal scan. PCOS can still be confirmed without a vaginal scan, and the prolactin test has nothing to do with marital status; it is simply a routine blood test. If a doctor confirms a prolactinoma and you need an MRI or dopamine-agonist treatment, this is usually managed at a hospital endocrine clinic, and a referral from the Klinik Kesihatan is the usual route to get there.

If you have just been through this process and feel lost among the row of blood tests, the quick-start guide for the newly diagnosed lays out the steps so you know what to ask at your next appointment.

What to ask your doctor

A little preparation before your appointment can make the visit more useful. Helpful questions to raise include:

  • “Were my prolactin and TSH checked before PCOS was confirmed?”
  • “If my prolactin is high, does it need repeating or a macroprolactin check before any conclusion?”
  • “Could any medicine I take now be raising my prolactin?”
  • “If the test keeps showing high prolactin, when would an MRI or specialist referral be needed?”

Bringing a current list of your medicines, including psychiatric medicines and supplements, helps a doctor assess accurately. Also note when your periods first became irregular and whether you have noticed galactorrhoea, because these details guide the investigation.

When to see a doctor

Get a doctor’s assessment if your periods are absent for more than 90 days without pregnancy, or if you notice milky discharge from the breast outside breastfeeding. Seek prompt care if galactorrhoea comes with severe, persistent headaches or vision changes, as this combination needs investigating without delay. If you take antipsychotics or antidepressants and your periods change, tell your doctor; do not stop those medicines on your own. The difference between PCOS and hyperprolactinaemia is not something you need to solve yourself. It is the doctor’s job to rule out other causes with the right tests, and your role is simply to make sure those tests are actually done.