Prolactin: The Test That Changes the Diagnosis

A man of 41 has low testosterone, no libido, and has been on gel for three years. It helped a little at first. He is still tired, his headaches have become routine, and last month he noticed he was catching the doorframe with his shoulder more often than seems normal.

Nobody ever measured his prolactin.

This is the one test where missing it does not just delay treatment. It substitutes the wrong treatment for years, while the actual cause continues doing what it does.

What prolactin does to testosterone

Prolactin is best known for milk production, which is why men rarely think about it. But it is produced in both sexes, and when it rises it acts on the brain rather than the testes.

High prolactin suppresses the kisspeptin neurons in the hypothalamus. Those neurons drive the pulsatile release of GnRH, which drives LH, which tells your testes to produce testosterone. Interrupt the pulse and the whole chain quietens.

The result is secondary hypogonadism: low testosterone with low or unremarkable LH. Your testes are fine. Nothing is telling them to work.

Elevated prolactin is among the most common causes of that pattern, which is why what causes low testosterone lists it, and why a proper workup includes it.

Why it gets missed in men

The tumour that causes it, a prolactinoma, is the most common type of functioning pituitary tumour, making up roughly 40 per cent of them.

In women they are usually found early, because raised prolactin disrupts periods and that gets noticed within months. In men there is no equivalent signal. Libido drops, energy falls, and both get attributed to work, age or stress.

So the tumours keep growing. By the time men are diagnosed, they typically have macroadenomas, larger tumours that have started pressing on nearby structures, with prolactin levels far higher than the microadenomas usually found in women.

That is the entire difference: not biology, but how long it takes anyone to look.

What it feels like

Two groups of symptoms, and the second group is what should trigger urgency.

From the hormone itself: low libido, erectile dysfunction, fatigue, low mood, reduced muscle mass, infertility, and over time reduced bone density. Indistinguishable from ordinary low testosterone, which is why the number is what identifies it rather than the symptoms.

From the tumour pressing on things:

• Headaches, often persistent and not typical of your usual pattern
• Visual changes, classically losing peripheral vision on both sides, because the optic nerves cross directly above the pituitary. Men describe clipping doorframes or not seeing cars approaching from the side
• Other pituitary hormones failing, producing thyroid or adrenal symptoms alongside

Occasionally milk production from the breasts, which is uncommon in men and worth mentioning if it happens.

Low testosterone plus headaches, or low testosterone plus any change in peripheral vision, is not a wait-and-see situation. That combination needs prolactin measured and, if raised, pituitary imaging.

What else raises prolactin

Most raised prolactin is not a tumour, and the alternatives are worth knowing before you worry.

Medication is the most common cause. Antipsychotics are the strongest offenders, particularly risperidone and older drugs. Some antidepressants, metoclopramide for nausea, and opioids all raise it. If you take any of these, that is the first explanation to consider.

An underactive thyroid. The signal that drives thyroid hormone also stimulates prolactin, so hypothyroidism raises both. This is why thyroid function belongs on the same panel.

Stress, exercise or nipple stimulation before the blood test can produce mild elevations. A modestly raised result should be repeated, rested, before anything else happens.

Chronic kidney disease reduces clearance.

So a slightly raised prolactin is a reason to look at your medication list and repeat the test. A substantially raised one is a reason for imaging.

Two laboratory traps

Both are technical, both cause real delays, and both are worth knowing because you may need to ask.

Macroprolactin. Prolactin can bind to antibodies forming a large complex that shows on the assay but is biologically inactive. This produces a raised result in a man with no problem at all. Laboratories can screen for it, and should when a raised prolactin does not fit the clinical picture.

The hook effect. This one runs the other way and is more dangerous. When prolactin is extremely high, the assay can saturate and report a falsely low or normal result. A man with a large tumour and enormous prolactin gets a reassuring number.

The fix is a diluted sample. If someone has a pituitary tumour on imaging and a prolactin that seems too modest for its size, ask specifically whether the sample was diluted.

The part that matters most

Here is why this test changes everything: prolactinomas are treated with tablets, not surgery.

Dopamine agonists, usually cabergoline, both suppress prolactin and shrink the tumour. It is one of the few brain tumours where medication is first-line and usually sufficient.

The results are good. Cabergoline normalises prolactin in around 80 per cent of men, with tumour shrinkage, improved visual fields and recovery of the hormonal axis in most.

For smaller tumours the numbers are better still. In a cohort of 47 men with microprolactinomas, 89 per cent achieved normal prolactin within a median of four months, and had normal testosterone at final follow-up. Side effects occurred in 4 per cent and settled with a dose reduction.

Read what that means. A man treated for the actual cause often recovers his own testosterone production, rather than replacing it externally for life. Most men who normalise prolactin recover their hormonal axis within twelve months.

Cabergoline also improves the things that brought him in: sexual function in around 60 per cent, sperm parameters in nearly all, and body composition, insulin resistance and waist circumference over the longer term.

Where testosterone still fits

Not every man recovers, and this deserves saying honestly.

Over 20 per cent of men with prolactinomas remain hypogonadal even after prolactin normalises, usually because a large tumour damaged the pituitary cells producing LH, or because suppression lasted long enough to cause lasting harm.

Those men do need testosterone, and giving it is correct. The men most likely to recover have smaller tumours, higher testosterone at diagnosis, no visual field defects and no other pituitary hormone deficiencies.

The distinction is sequence. Treat the prolactin first, wait to see what recovers, then replace what does not. Starting testosterone without ever measuring prolactin gets that order exactly backwards.

And if fertility matters, testosterone is the wrong drug anyway. Men needing fertility after prolactinoma treatment are given gonadotropins or clomiphene instead. TRT and fertility covers why.

When to ask for it

Prolactin belongs in any proper low testosterone workup. Ask specifically if:

• Your LH is low or normal alongside low testosterone. This is the pattern that points upstream
• You have headaches that are new or persistent
• You have noticed any change in peripheral vision
• You take antipsychotics, certain antidepressants, metoclopramide or opioids
• Your testosterone is very low without an obvious cause
• You have been on testosterone for a while and still feel unwell

That last one is the situation this article opened with, and it is more common than it should be. If prolactin was never checked before you started, it is not too late to check it now. Online TRT clinics covers what a proper assessment includes, and research presented in 2026 found only 12 per cent of prescriptions met that standard.

The honest summary

Most men with low testosterone do not have a prolactinoma. It is uncommon.

But it is the one cause where the correct treatment is entirely different from testosterone, where that treatment is a tablet with an 80 to 90 per cent success rate, and where delay means a tumour keeps growing next to your optic nerves.

The test costs very little and takes one blood sample. If your LH is low and nobody has measured your prolactin, ask.

Common questions

Why would low testosterone need a prolactin test?

Because high prolactin suppresses the brain signal that tells your testes to produce testosterone. It is among the most common causes of low testosterone with low or normal LH, and it is treated completely differently.

What is a prolactinoma?

A benign pituitary tumour that produces prolactin, and the most common functioning pituitary tumour. In men it is usually found late and therefore larger, because there is no early symptom equivalent to the disrupted periods that flag it in women.

Does it need surgery?

Usually not. Dopamine agonists such as cabergoline are first-line, normalising prolactin in around 80 per cent of men and shrinking the tumour. Surgery is reserved for resistant cases.

Will my testosterone recover?

Often. Most men recover their hormonal axis within twelve months of prolactin normalising, and in one microprolactinoma cohort 89 per cent achieved normal prolactin and testosterone. Over 20 per cent remain hypogonadal and do need replacement.

What if my prolactin is only slightly raised?

Review your medication first, particularly antipsychotics, some antidepressants, metoclopramide and opioids, and check thyroid function. Repeat the test rested, since stress and exercise cause mild elevations.

Can a prolactin test be wrong?

Yes, in both directions. Macroprolactin causes falsely high results, and the hook effect can cause falsely low results when prolactin is extremely high. If a tumour is visible but prolactin seems too low for its size, ask whether the sample was diluted.

Sources: Petersenn S et al., Approach to the patient with prolactinoma, J Clin Endocrinol Metab 2023;108(9):2400-23 · Shimon I, Prolactinomas in males: any differences?, Pituitary 2020;23(1):52-7 · Response to cabergoline treatment, gonadal axis recovery, and outcomes of drug withdrawal in men with microprolactinoma: a retrospective cohort study, Pituitary 2025 · Hypogonadism in men with prolactinoma: diagnosis, treatment, and management of persistent hypogonadism, 2025 · Educational only, not medical advice.

Keep reading: What causes low testosterone · Normal blood test, still low testosterone · Blood test: what to check · Online TRT clinics · TRT and fertility · Testosterone and bone health