High Prolactin: Do You Need a Pituitary MRI?

Prolactin is a hormone produced by the pituitary gland, primarily known for its role in breast milk production. But prolactin does more than that — and when levels are abnormally high (hyperprolactinaemia), it can cause symptoms in both women and men. The most common structural cause is a prolactinoma — a benign pituitary tumour — and MRI of the pituitary gland is the test that finds it.

What's a Normal Prolactin Level?

Normal ranges vary between labs, but generally:

• Women (non-pregnant): Less than 500 mU/L (or less than 25 ng/mL)
• Men: Less than 400 mU/L (or less than 20 ng/mL)
• Pregnancy: Prolactin rises dramatically and can exceed 6,000 mU/L by the third trimester — this is normal

The degree of elevation matters:

• Mildly elevated (500–1,000 mU/L): Multiple possible causes including medication, stress, and non-prolactin-producing pituitary tumours
• Moderately elevated (1,000–5,000 mU/L): Microprolactinoma becomes likely
• Highly elevated (above 5,000 mU/L): Macroprolactinoma is probable
• Very highly elevated (above 10,000 mU/L): Almost certainly a macroprolactinoma

Symptoms of High Prolactin

In women

• Menstrual irregularity or absence (amenorrhoea): High prolactin suppresses GnRH, which disrupts the normal menstrual cycle
• Galactorrhoea: Milk production from the breasts outside of pregnancy and breastfeeding
• Reduced fertility: Disrupted ovulation due to hormonal imbalance
• Reduced libido: From the downstream hormonal effects
• Vaginal dryness: Due to low oestrogen
• Bone density loss: Long-term oestrogen deficiency from chronic hyperprolactinaemia can reduce bone density

In men

• Erectile dysfunction: One of the most common presentations
• Reduced libido: From low testosterone (high prolactin suppresses testosterone production)
• Gynaecomastia: Breast tissue enlargement
• Galactorrhoea: Less common in men but possible
• Reduced muscle mass and energy: From testosterone deficiency
• Infertility: Low testosterone and disrupted sperm production

In both sexes

• Headaches: If a pituitary tumour is large enough to cause pressure
• Visual field changes: A large pituitary tumour can press on the optic chiasm (where the optic nerves cross), causing loss of peripheral vision — typically in both outer visual fields (bitemporal hemianopia)

Causes of High Prolactin

Medications (the most common cause)

Many commonly prescribed medications raise prolactin:

• Antipsychotics: Risperidone, haloperidol, olanzapine, quetiapine — these are the biggest offenders. Risperidone can raise prolactin above 2,000 mU/L.
• Antidepressants: SSRIs and tricyclics can cause mild elevation
• Anti-emetics: Metoclopramide, domperidone — commonly used for nausea
• Opioids: Chronic opioid use raises prolactin
• Some antihypertensives: Methyldopa, verapamil

If medication is the likely cause, your doctor may recheck prolactin after stopping or switching the medication (where clinically safe). If levels remain high, MRI is indicated.

Prolactinoma

A prolactinoma is a benign (non-cancerous) tumour of the prolactin-producing cells in the pituitary gland. They're the most common type of pituitary tumour and are classified by size:

• Microprolactinoma: Less than 10mm. More common in women. Often found because of menstrual irregularity or fertility problems. Prolactin is usually 1,000–5,000 mU/L.
• Macroprolactinoma: 10mm or larger. More common in men (because symptoms in men tend to be subtler and present later, allowing the tumour to grow larger before diagnosis). Prolactin is usually above 5,000 mU/L and can be extremely high.

Other pituitary tumours

Non-prolactin-producing pituitary tumours can raise prolactin by compressing the pituitary stalk — the connection between the hypothalamus and the pituitary gland. This is called "stalk effect" or "disconnection hyperprolactinaemia." The prolactin level in stalk effect is typically below 2,000 mU/L — an important distinction from prolactinomas, where levels are usually higher.

Other causes

• Hypothyroidism: High TSH stimulates prolactin production. Always check thyroid function.
• PCOS: Mildly elevated prolactin is found in some women with polycystic ovary syndrome.
• Stress: Significant stress, including the stress of venepuncture itself, can mildly elevate prolactin.
• Macroprolactinaemia: A laboratory artefact where prolactin molecules form large complexes. The measured level is high, but the biologically active prolactin is normal. This is harmless but can cause unnecessary investigation if not recognised.
• Chronic kidney disease: Reduced clearance of prolactin.
• Chest wall irritation: Herpes zoster (shingles) affecting the chest, or even chest wall surgery, can occasionally raise prolactin through neural pathways.

The Pituitary MRI

A pituitary MRI is a focused scan of the sella turcica — the small bony pocket where the pituitary gland sits. It's different from a routine brain MRI in that it uses:

• Thin slices: 2–3mm sections through the pituitary (a standard brain MRI uses 4–5mm slices, which could miss a small tumour)
• Dynamic contrast: Rapid imaging during gadolinium injection shows how the normal pituitary and any tumour enhance differently — a microadenoma typically enhances more slowly than normal pituitary tissue
• Multiple planes: Coronal and sagittal views to show the tumour's relationship to surrounding structures

What the MRI shows

• Tumour size and location: Micro or macro, exact dimensions, position within the gland
• Cavernous sinus invasion: Whether the tumour has extended into the cavernous sinuses (the venous structures on either side of the pituitary), which affects surgical planning
• Optic chiasm relationship: Whether the tumour is pressing on or displacing the optic nerves
• Sphenoid sinus extension: Whether the tumour extends downward into the sinus below
• Normal pituitary tissue: How much normal gland is compressed and whether other pituitary functions might be affected

Treatment of Prolactinomas

The good news: prolactinomas are one of the most treatable pituitary tumours. Unlike most tumours, the first-line treatment is medication, not surgery.

Dopamine agonists

Cabergoline and bromocriptine are medications that mimic dopamine, which naturally suppresses prolactin production. They work remarkably well:

• Normalise prolactin in over 80% of patients
• Shrink the tumour — often dramatically — in most cases
• Restore menstrual function and fertility in women
• Improve testosterone levels and sexual function in men

Cabergoline is generally preferred over bromocriptine because it's taken less frequently (usually once or twice a week) and has fewer side effects.

Surgery

Transsphenoidal surgery (through the nose) is reserved for patients who:

• Don't respond to or can't tolerate dopamine agonists
• Have very large tumours causing urgent visual compromise
• Have tumours with atypical features on MRI
• Prefer a surgical solution

Monitoring

Whether treated with medication or surgery, follow-up MRI is used to monitor tumour size. For microprolactinomas responding well to medication, MRI might be repeated annually for the first few years, then less frequently if stable.

When to Get a Pituitary MRI

Consider a pituitary MRI if:

• Blood tests show persistently elevated prolactin after excluding medication causes and macroprolactinaemia
• You have symptoms consistent with high prolactin (menstrual irregularity, galactorrhoea, erectile dysfunction, reduced libido)
• You have headaches combined with visual disturbance (especially peripheral vision loss)
• Other pituitary hormones are abnormal (suggesting a larger pituitary tumour affecting multiple cell types)
• You're being monitored for a known prolactinoma

Prolactinomas are common, benign, and treatable. But they need to be found first. Pituitary MRI is the definitive test — it shows exactly what's there, how big it is, and what it's affecting. With that information, treatment decisions are straightforward.