Prolactinoma — UKMLA Guide 2026

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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.

Key points

Prolactinoma = most common functioning pituitary adenoma (~40% of all pituitary tumours)
Common causes of hyperprolactinaemia: prolactinoma, drugs (antipsychotics, metoclopramide), pregnancy, hypothyroidism, stalk effect
Clinical: galactorrhoea + amenorrhoea/oligomenorrhoea (women), erectile dysfunction + reduced libido (men), infertility, osteoporosis
Diagnosis: serum prolactin (>5000 mU/L strongly suggests macroprolactinoma). Check macroprolactin. MRI pituitary
First-line treatment: dopamine agonist (cabergoline preferred over bromocriptine) — NOT surgery. Effective in >80%
Large macroadenoma: "hook effect" — falsely normal prolactin with very large tumours. Request serial dilution

Overview

Prolactinoma is the most common functioning pituitary tumour, accounting for approximately 40% of all pituitary adenomas. They are classified by size: microprolactinoma (<10 mm) and macroprolactinoma (≥10 mm). Prolactin level broadly correlates with tumour size. Microprolactinomas are far more common in women, while macroprolactinomas have more equal sex distribution. Hyperprolactinaemia can also result from non-tumour causes: drugs that block dopamine (antipsychotics, metoclopramide, domperidone), hypothyroidism (TRH stimulates prolactin), pregnancy/lactation, and stalk effect (non-functioning pituitary tumour compresses the stalk, reducing dopamine delivery to lactotrophs). Prolactin inhibits GnRH pulsatility, leading to hypogonadism and infertility.

Epidemiology

Prolactinomas have a prevalence of approximately 50 per 100,000 population. Microprolactinomas are far more common in women (F:M ~10:1, age 20–40 years) — often detected early due to menstrual disturbance. Macroprolactinomas have a more equal sex distribution — men present later with mass effect symptoms because they may not notice hypogonadal symptoms early. Drug-induced hyperprolactinaemia is the most common overall cause of raised prolactin — always take a drug history before investigating further.

Clinical Features

Symptoms

Galactorrhoea (inappropriate breast milk production — may be bilateral)

Women: amenorrhoea/oligomenorrhoea, infertility, reduced libido

Men: erectile dysfunction, reduced libido, gynaecomastia, infertility (men often present late)

Headache (macroadenoma — mass effect)

Visual field loss (bitemporal hemianopia — chiasm compression by macroadenoma)

Features of hypopituitarism (fatigue, hypothyroid, adrenal insufficiency — from mass effect on normal pituitary)

Signs

Galactorrhoea on breast examination

Bitemporal hemianopia (macroadenoma)

Features of hypogonadism: reduced secondary sexual characteristics, osteoporosis

Gynaecomastia in men

Investigations

First-line

Serum prolactinMildly raised (up to 2000 mU/L) = drugs, stalk effect, stress. Moderately raised (2000–5000) = microprolactinoma or stalk effect. >5000 mU/L = macroprolactinoma very likely

Pregnancy testExclude pregnancy in any woman of reproductive age with raised prolactin

Drug historyAntipsychotics, metoclopramide, domperidone, SSRIs, opioids — common causes of mild-moderate elevation

Second-line

TFTsExclude hypothyroidism (high TRH stimulates prolactin)

MacroprolactinRequest if mildly elevated prolactin with no symptoms — macroprolactin is biologically inactive and a false positive

MRI pituitary with gadoliniumConfirm and size the adenoma. If large tumour but modest prolactin rise → suspect stalk effect (not a prolactinoma)

Pituitary function testsFull anterior pituitary profile: LH, FSH, testosterone/oestradiol, cortisol, TFTs, IGF-1, GH

Specialist

Visual field testingFormal perimetry for all macroadenomas near/compressing the optic chiasm

DEXA scanIf prolonged hypogonadism — assess for osteoporosis

Serial dilution of prolactinIf large macroadenoma with unexpectedly normal/low prolactin → "hook effect" (assay saturation). Diluted sample reveals true high level

Management

Endocrine Society Clinical Practice Guideline — Hyperprolactinaemia, 2011 (reaffirmed)

First-line: dopamine agonist therapy

Cabergoline 0.25–0.5 mg twice weekly (preferred — more effective, fewer side effects, better tumour shrinkage than bromocriptine)
Bromocriptine 1.25–15 mg daily (alternative, cheaper, shorter acting)
Normalises prolactin in >80–90%, reduces tumour size by >50% in ~70%
Side effects: nausea, dizziness, postural hypotension. Rare: impulse control disorders (gambling, hypersexuality), valvular fibrosis (high doses — echocardiogram if prolonged use)
Monitor: prolactin 1 month post-start, then 3–6 monthly. MRI at 12 months, then annually if macroadenoma

Drug-induced hyperprolactinaemia

If possible: switch antipsychotic to prolactin-sparing agent (quetiapine, aripiprazole, clozapine) — only with psychiatrist input
If cannot stop drug: manage symptoms (HRT for hypogonadism, fertility treatment if needed)
Do NOT add cabergoline to an antipsychotic without psychiatric consultation (risk of psychotic relapse)

Surgery (transsphenoidal)

Second-line if: dopamine agonist-resistant (rare), intolerant, cystic tumour not responding, patient preference
Surgical cure rate: ~90% for microprolactinomas, ~50% for macroprolactinomas
Post-operative recurrence: 20–50% — may need to resume dopamine agonist

Special situations

Pregnancy: stop cabergoline once pregnancy confirmed (microprolactinoma risk of growth is low ~2%). If macroadenoma: closer monitoring, visual fields each trimester
Microprolactinoma: can trial withdrawal of cabergoline after 2+ years if prolactin normalised AND tumour shrunk — ~30% achieve lasting remission

Complications

Infertility: Prolactin inhibits GnRH → hypogonadotrophic hypogonadism → anovulation/azoospermia. Reversible with treatment
Osteoporosis: From prolonged hypogonadism — DEXA monitoring, consider HRT or testosterone if not planning fertility
Visual loss: Macroadenoma compressing optic chiasm — urgent dopamine agonist therapy shrinks tumour
Pituitary apoplexy: Rare — haemorrhage or infarction of adenoma. Acute headache, visual loss, cranial nerve palsies, haemodynamic instability. Neurosurgical emergency
Hypopituitarism: From mass effect of large tumour on normal pituitary — may need cortisol, thyroxine, sex hormone replacement

UKMLA Exam Tips

1Dopamine agonist is FIRST-LINE for prolactinoma — NOT surgery (unlike most other pituitary tumours)
2Cabergoline > bromocriptine (more effective, better tolerated, less frequent dosing)
3Drug-induced hyperprolactinaemia: most common overall cause. ALWAYS check drug history before ordering MRI
4Stalk effect: non-functioning pituitary tumour compresses stalk → reduced dopamine delivery → mild prolactin rise (<2000 mU/L). NOT a prolactinoma
5Hook effect: VERY large tumour + falsely normal prolactin due to assay saturation. Request serial dilution
6Prolactin level correlates with tumour size. A 3 cm tumour with prolactin of 800 is NOT a prolactinoma (it's stalk effect)
7Men present later than women (no periods to miss) — larger tumours and more mass effect at diagnosis

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