Hyperprolactinaemia and Thyroid Disorders: What Connects Them and What To Do

• Primary hypothyroidism is the commonest thyroid cause of high prolactin: TRH rises, pushing both TSH and prolactin up.
• Always check TSH and free T4 with a raised prolactin before jumping to pituitary MRI.
• Treating hypothyroidism with levothyroxine usually normalises prolactin within 6-12 weeks.
• Prolactin >200 ng/mL (≈4,200 mIU/L) strongly suggests a prolactinoma; 25-100 ng/mL is often medication, stress, or thyroid related.
• Confirm true elevation: repeat a calm, morning sample; screen for macroprolactin; consider the “hook effect” in big pituitary masses.

Half the “high prolactin” results I see aren’t tumours at all-they’re a slow thyroid calling the shots. If you get the thyroid right, prolactin often falls, periods return, and the MRI can wait. Here’s how the two hormones talk to each other, what to test, how to read the numbers, and the treatment paths that actually work.

The thyroid-prolactin link in plain terms

Start with the signal caller: thyrotropin-releasing hormone (TRH). When your thyroid runs slow (primary hypothyroidism), the brain pushes TRH up to drive more TSH. TRH doesn’t just nudge TSH-it also stimulates lactotroph cells in the pituitary, which raises prolactin. That’s the cleanest explanation for why hypothyroidism and hyperprolactinaemia so often show up together.

Common symptoms overlap. In women, that can mean irregular or absent periods, trouble getting pregnant, hot flashes, low libido, and sometimes milk discharge (galactorrhoea). In men, think low libido, erectile dysfunction, low energy, and reduced shaving frequency. Add classic hypothyroid features-weight gain, cold intolerance, dry skin, constipation-and the picture starts to make sense.

What about an overactive thyroid (hyperthyroidism)? TRH is usually suppressed, so prolactin tends to be normal or even low. If prolactin is a bit high with hyperthyroidism, it’s often from stress, medicines, or lab noise, not the thyroid itself. Postpartum thyroiditis is a special case: prolactin is naturally high in late pregnancy and while breastfeeding, so don’t over-interpret mild prolactin bumps during that window.

One more curveball: long-standing primary hypothyroidism can cause the pituitary to look big on MRI (thyrotroph hyperplasia) and mildly raise prolactin. It’s not a tumour-and it shrinks once you replace thyroid hormone.

Evidence to back this up? This link is standard endocrine physiology and is reflected in major guidance: Endocrine Society hyperprolactinaemia clinical practice guideline (2011, with updates and the 2023 Pituitary Society consensus), American Thyroid Association guidelines for hypothyroidism (2014) and pregnancy (2017), and NICE thyroid disease guidance (2019, updated 2023).

What to test, when to test, and how to prepare

Before ordering an MRI, make your labs count. A smart, simple sequence saves time, money, and a scare.

Step-by-step labs:

1. Take a calm, morning prolactin. Sit quietly for 15 minutes before the draw. Avoid nipple stimulation, heavy exercise, or sexual activity the night before. If you’re nervous, ask for a repeat in a quiet setting.
2. With that prolactin, always check thyroid function: TSH and free T4. This is non-negotiable with any raised prolactin.
3. Rule out the obvious physiologic causes: pregnancy test (if relevant), breastfeeding, sleep deprivation, acute stress, chest wall stimulation or injury.
4. Medicines review: write down all meds and supplements. Common culprits that raise prolactin include antipsychotics (risperidone, haloperidol), metoclopramide and domperidone, verapamil, methyldopa, high-dose oestrogens, and opioids. Combined hormonal contraceptives can blur the picture.
5. If prolactin is only mildly elevated (roughly 25-100 ng/mL or 530-2,100 mIU/L) and symptoms are minimal, ask the lab to screen for macroprolactin. Macroprolactin is a big, mostly inactive form that can falsely read high.
6. If there’s a big pituitary lesion on imaging but prolactin is oddly modest, ask the lab to run a dilution series to check for the “hook effect” (very high prolactin overwhelming the assay).

Rules of thumb for the numbers (labs differ slightly):

• Normal prolactin: about 5-20 ng/mL (≈100-400 mIU/L).
• Stress or lab artefact: up to 40-60 ng/mL is common with a bad draw, anxiety, or recent breast stimulation.
• Hypothyroidism, medicines, chest wall causes: often 25-100 ng/mL, sometimes higher.
• Prolactinoma: >100 ng/mL is suspicious; >200 ng/mL (≈4,200 mIU/L) is very suggestive, especially if thyroid and medicines are ruled out.

When to order pituitary MRI:

• Prolactin >100 ng/mL on a good-quality, repeated sample with normal thyroid tests and no culprit medicines.
• Any prolactin elevation plus visual changes, new severe headaches, or neurological findings.
• A pituitary mass found incidentally with prolactin above the lab’s upper limit, after excluding hypothyroidism and macroprolactin.

Special situations:

• Pregnancy and breastfeeding: prolactin is naturally high; avoid diagnosing hyperprolactinaemia unless symptoms are severe. Imaging is reserved for red flags (visual field loss, severe headaches).
• Trying to conceive: even mild prolactin elevation can disrupt ovulation. Check thyroid early and treat hypothyroidism; aim for a TSH target individualised to context (often below 2.5 mIU/L in preconception care per ATA 2017).
• On antipsychotics: never stop or switch on your own. Work with psychiatry. Aripiprazole sometimes lowers prolactin when added or substituted.

How to read your results and common scenarios

Here’s how I talk patients through typical lab combinations and what they usually mean.

Scenario 1: Prolactin 35 ng/mL; TSH 9 mIU/L; free T4 low.

• Likely cause: primary hypothyroidism causing a mild prolactin rise via TRH.
• Plan: start levothyroxine, titrate to normalise TSH and free T4. Recheck prolactin in 6-12 weeks. Most see prolactin normalise without dopamine agonists.

Scenario 2: Prolactin 220 ng/mL; TSH 2.0 mIU/L; no culprit meds; amenorrhoea and galactorrhoea.

• Likely cause: prolactinoma (pituitary adenoma secreting prolactin).
• Plan: pituitary MRI; if confirmed, start cabergoline unless contraindicated. Visual symptoms or apoplexy? Urgent endocrine and neurosurgical input.

Scenario 3: Prolactin 80 ng/mL; TSH normal; taking risperidone.

• Likely cause: medicine-induced hyperprolactinaemia.
• Plan: discuss options with psychiatry-dose adjustment, switch to a prolactin-sparing antipsychotic, or add-on aripiprazole. Endocrine meds (dopamine agonists) are a last resort here due to psychiatric risks.

Scenario 4: Prolactin 120 ng/mL; MRI shows enlarged pituitary; TSH 35 mIU/L; free T4 low.

• Likely cause: thyrotroph hyperplasia from severe hypothyroidism, not a true adenoma.
• Plan: treat hypothyroidism and repeat MRI in 3-6 months. The gland often shrinks; prolactin falls with treatment.

Scenario 5: Prolactin 45 ng/mL; TSH 3.8 mIU/L; free T4 normal; trying to conceive.

• Possible: subclinical hypothyroidism with mild prolactin bump, stress artefact, or macroprolactin.
• Plan: repeat a calm-sample prolactin with macroprolactin screen; consider levothyroxine if TSH persistently elevated in the fertility context after shared decision-making (ATA 2017 suggests tighter TSH targets preconception).

Key pitfalls to avoid:

• Chasing an MRI before checking TSH and free T4.
• Missing macroprolactin when symptoms don’t match the lab number.
• Forgetting the hook effect in a large pituitary mass with only mildly raised prolactin-ask for serial dilutions.
• Drawing prolactin right after a stressful commute, vigorous exercise, or a breast exam.

Fertility and bone health considerations:

• High prolactin suppresses GnRH, which lowers oestrogen/testosterone, reducing bone density over time. Thyroid dysfunction also affects bone-hyperthyroidism speeds bone turnover; overt hypothyroidism can slow bone formation and raise lipids.
• Fixing the thyroid and prolactin restores sex hormone balance, improving cycles, sperm parameters, and protecting bone. If hypogonadism persists after prolactin normalises, reassess.

Treatment playbook by cause

Primary hypothyroidism with raised prolactin:

• Start levothyroxine. Dose is weight- and age-dependent; your clinician will set it and monitor TSH and free T4 every 6-8 weeks until stable.
• Expect prolactin to fall as thyroid normalises, often within 6-12 weeks. Dopamine agonists are rarely needed unless symptoms are severe or fertility is urgent.
• If galactorrhoea persists after normal thyroid and prolactin, ensure no nipple stimulation and consider a short course of a dopamine agonist only if distressing.

Prolactinoma (micro- or macro-):

• Cabergoline is first-line in most cases (Pituitary Society consensus 2023). Typical start: 0.25 mg twice weekly, titrate by symptoms and prolactin levels.
• Bromocriptine is preferred in pregnancy planning or when cabergoline isn’t tolerated. It has the longest safety record in pregnancy.
• Monitor: prolactin monthly until normal, then every 3-6 months; MRI at 6-12 months (earlier if macroadenoma or visual symptoms). Visual fields if the tumour is near the optic chiasm.
• Side effects: nausea, dizziness, nasal congestion; take with food, dose at night, titrate slowly. Valve disease risk with cabergoline at low endocrine doses appears small; baseline echocardiogram is considered for higher cumulative doses or cardiac history.
• Surgery is for dopamine agonist resistance/intolerance, apoplexy, CSF leak, or rapid visual compromise.

Medicine-induced hyperprolactinaemia:

• First choice: switch to or add a prolactin-sparing agent (e.g., aripiprazole) after discussion with the prescribing specialist.
• If a change isn’t possible and symptoms are significant, endocrinology may consider low-dose dopamine agonist with careful psychiatric oversight.

Hyperthyroidism with a raised prolactin:

• Fix the thyroid first (antithyroid drugs, radioactive iodine, or surgery per ATA 2016). The prolactin usually settles if it was noise or stress-related.
• If prolactin stays high after euthyroidism is achieved, repeat the workup as above.

Pregnancy and postpartum:

• Prolactin rises in pregnancy and during lactation; don’t over-interpret.
• Prolactinomas often shrink on dopamine agonists, but in pregnancy, bromocriptine is the go-to if treatment is needed. Many women with microprolactinomas stop dopamine agonists during pregnancy and do fine with monitoring.
• Hypothyroidism in pregnancy requires prompt levothyroxine and tighter TSH goals to protect mother and baby (ATA 2017).

Lifestyle levers (useful but not a cure for pathological causes):

• Sleep: aim for 7-9 hours; sleep loss raises stress hormones that can nudge prolactin.
• Stress: breathing drills, short walks, or brief mindfulness before the blood draw can prevent a false bump.
• Avoid nipple stimulation before testing; schedule breast exams after, not before, your bloods.

Checklists, examples, and pro tips

Quick checklist: what to do after a raised prolactin result

1. Repeat prolactin in the morning, seated for 15 minutes, in a calm setting.
2. Order TSH and free T4 with the repeat sample.
3. Review medicines; ask your clinician if any are known to raise prolactin.
4. Consider a pregnancy test if relevant.
5. If mild elevation with few symptoms, ask for a macroprolactin screen.
6. If prolactin stays >100 ng/mL with normal thyroid and no culprit meds, book a pituitary MRI.

Interpretation cheat sheet

• 25-60 ng/mL: repeat under ideal conditions; check thyroid; think stress, meds, macroprolactin.
• 60-100 ng/mL: check thyroid and medicines; macroprolactin screen; MRI if persistent and symptomatic.
• >100 ng/mL: exclude hypothyroidism and meds; MRI likely needed.
• >200 ng/mL: prolactinoma until proved otherwise.

Two real-world style examples

• Young woman with missed periods and milky discharge: prolactin 58 ng/mL on a rushed afternoon draw; TSH 10 mIU/L. Repeat morning prolactin 32 ng/mL; start levothyroxine. Six weeks later, TSH normal, prolactin 14 ng/mL, periods back.
• Middle-aged man with low libido and headaches: prolactin 280 ng/mL; thyroid normal. MRI shows a 12 mm pituitary adenoma. Cabergoline started; prolactin normalises in 4 weeks; tumour shrinks at 3 months; libido returns.

Pro tips

• Draw blood before a breast exam or mammogram, not after.
• Ask the lab about macroprolactin if your symptoms are mild but the number is high.
• If you see a big pituitary and modest prolactin, request serial dilution to rule out the hook effect.
• If you’re trying to conceive, a normal thyroid can be the difference between irregular and ovulatory cycles. Don’t skip the TSH.

FAQs and next steps

Does high prolactin cause thyroid disease?

Not typically. The traffic mainly goes the other way: hypothyroidism raises TRH, which lifts prolactin. There are hints that prolactin can modulate immunity and might nudge thyroid autoimmunity in some people, but that’s not the main clinical story.

Can levothyroxine raise prolactin?

No. It usually lowers prolactin by turning down TRH once the thyroid is replaced properly.

My prolactin is high but I feel fine. Could it be harmless?

It could be macroprolactin. That form is large and biologically quiet. Ask for a macroprolactin assay (polyethylene glycol precipitation). If macroprolactin predominates and you’re symptom-free, many clinicians simply observe.

Do I need to stop my antipsychotic if prolactin is high?

Don’t stop on your own. Work with the prescriber. Options include dose adjustments, switching to a prolactin-sparing medicine, or adding aripiprazole. Endocrine medication is a backup plan.

Will a prolactinoma affect my fertility?

Yes, until treated. Dopamine agonists often restore ovulation and sperm quality. Many people conceive naturally once prolactin normalises.

How fast should prolactin fall after treating hypothyroidism?

Often within 6-12 weeks as TSH and free T4 normalise. If it doesn’t, revisit meds, macroprolactin, stress at sampling, and consider imaging if numbers remain high.

Is cabergoline safe for the heart valves?

At the low doses used for prolactinomas, valve risk appears small. Your clinician may consider a baseline echocardiogram if you need higher cumulative doses or have cardiac history (Pituitary Society 2023).

Next steps by situation

• If you’re trying to conceive: check TSH, free T4, and prolactin early; treat hypothyroidism promptly; consider targeting a lower TSH after shared decision-making; treat prolactin causes to restore ovulation/spermatogenesis.
• If you’re on medicines that raise prolactin: book a review with the prescriber to discuss alternatives; don’t discontinue abruptly.
• If your prolactin is very high (>200 ng/mL): arrange pituitary MRI and see endocrinology. If visual changes or severe headache, go urgent.
• If you’re postpartum/breastfeeding: defer prolactin interpretation unless symptoms are marked; focus on thyroid screening if you have fatigue, mood change, or cycle issues after weaning.
• If your MRI shows a big pituitary but prolactin is only mildly elevated: ask the lab for dilution studies (hook effect) and confirm thyroid status before any surgery talk.

Credible sources behind this guidance

• Endocrine Society Clinical Practice Guideline: Diagnosis and Treatment of Hyperprolactinaemia (2011)
• Pituitary Society Consensus Statement on Prolactinomas (2023)
• American Thyroid Association Guidelines for Hypothyroidism (2014) and Hyperthyroidism (2016)
• American Thyroid Association Guidelines for Thyroid Disease in Pregnancy (2017)
• NICE Thyroid Disease: Assessment and Management (NG145, 2019; updates through 2023)

If you remember one thing, let it be this: in any raised prolactin, check the thyroid first. It’s the simplest fix, and it spares a lot of unnecessary scans and stress.