Hyperprolactinaemia Interpreter (High Prolactin)
Hyperprolactinaemia Interpreter (High Prolactin)
Enter a prolactin in mIU/L or µg/L with your laboratory’s upper limit, and what is known — macroprolactin screening, pregnancy, drugs, hypothyroidism, renal failure, pituitary imaging. It reads the level against the thresholds the Endocrine Society and Pituitary Society use, and flags the two traps that change management: macroprolactin, and a large tumour with a modest prolactin that may be a stalk effect or a hook effect.
High prolactin
Prolactin + context → causeProlactin 1,800 mIU/L, upper limit 500 mIU/L, macroadenoma of 10 mm or more on MRI, no drugs, not pregnant
Conversion and thresholds
- 21.2
- mIU per µg against the WHO 3rd International Standard 84/500
- × ULN
- result ÷ your laboratory’s upper limit; below 5 × ULN, the 2023 consensus recommends repeating
- 94 µg/L
- above this, prolactinoma rather than a nonfunctioning macroadenoma (series cited by the 2011 guideline)
- 250 and 500 µg/L
- usually a prolactinoma; diagnostic of a macroprolactinoma (Endocrine Society 2011)
Worked example
Prolactin 1,800 mIU/L, upper limit 500 mIU/L, macroadenoma of 10 mm or more on MRI, no drugs, not pregnant
1,800 ÷ 21.2 = 84.9 µg/L; 1,800 ÷ 500 = 3.6 × the upper limit
A macroadenoma with prolactin at or below 94 µg/L: stalk effect or hook effect
Stalk effect: a nonfunctioning adenoma compressing the stalk; the 2023 consensus allows up to about 6 × ULN
Hook effect: a large prolactinoma reading falsely low — ask for a 1:100 dilution
Below 5 × ULN, and macroprolactin not screened: the page also flags a repeat and a PEG screen
The thresholds, and where each comes from
| Prolactin | mIU/L (× 21.2) | What it suggests | Source |
|---|---|---|---|
| 25–100 µg/L | 530–2,120 | The usual range of a drug effect | Endocrine Society 2011 |
| Above 94 µg/L with a macroadenoma | Above 1,993 | Prolactinoma rather than a nonfunctioning adenoma | Series of 226 adenomas cited in the 2011 guideline |
| Up to about 6 × ULN | — | Compatible with a stalk effect | Pituitary Society 2023 |
| Below 5 × ULN | — | Repeat the test before investigating | Pituitary Society 2023 (strong) |
| Below 200 µg/L | Below 4,240 | Check for macroprolactin | Pituitary Society 2023 |
| Above 200 µg/L | Above 4,240 | Usually diagnostic of a prolactinoma — but risperidone and metoclopramide can exceed it | Pituitary Society 2023; Endocrine Society 2011 |
| Above 250 µg/L | Above 5,300 | Usually a prolactinoma; typical of macroprolactinomas | Endocrine Society 2011 |
| Above 500 µg/L | Above 10,600 | Diagnostic of a macroprolactinoma | Endocrine Society 2011 |
| During pregnancy | — | Rises about ten-fold, 150–300 µg/L by term | Endocrine Society 2011 |
A single prolactin, read with the context that decides it
A raised prolactin is one of the commonest abnormal hormone results and one of the easiest to over-investigate. The 2011 Endocrine Society guideline makes the diagnosis on a single result above the upper limit, from a sample taken without excessive venepuncture stress, and then asks for the ordinary causes to be excluded before anyone reaches for an MRI: pregnancy, drugs, primary hypothyroidism, renal failure, and macroprolactin. The 2023 Pituitary Society consensus adds that a result below five times the upper limit should simply be repeated first.
The level itself carries information. Drug-induced rises usually sit between 25 and 100 µg/L; most people with a prolactin above 250 µg/L have a prolactinoma; and above 500 µg/L the diagnosis is a macroprolactinoma. This page converts a mIU/L result to µg/L at 21.2, the factor for the current WHO standard and the one the prolactin unit converter uses, because every published threshold is in µg/L.
Two traps run in opposite directions. Macroprolactin — prolactin bound to IgG — is measured by immunoassays but is largely inactive, and in retrospective series cited by the 2011 guideline it accounted for about 40% of hyperprolactinaemia; a PEG screen, and the macroprolactin PEG recovery calculator, settle it cheaply. The other is the large tumour with a modest prolactin. It may be a nonfunctioning adenoma compressing the stalk, which a dopamine agonist will not shrink, or a macroprolactinoma whose prolactin is so high that it saturates the assay and reads low — the hook effect. The fix is a 1:100 dilution, and a 2026 systematic review found the hook effect in tumours as small as 22 mm, not only in giant ones.
The headline is shown in µg/L with the mIU/L equivalent and the multiple of your upper limit beneath it. For the growth hormone axis, see the IGF-1 interpreter.
Frequently asked questions
What prolactin level suggests a prolactinoma?
The 2011 Endocrine Society guideline says a prolactin above 250 µg/L (about 5,300 mIU/L) usually indicates a prolactinoma and above 500 µg/L (about 10,600 mIU/L) is diagnostic of a macroprolactinoma. The 2023 Pituitary Society consensus puts 200 µg/L or more as usually diagnostic. Lower levels have many causes, and drugs such as risperidone and metoclopramide can exceed 200 µg/L.
How do I convert prolactin from mIU/L to µg/L?
Divide by 21.2. That factor comes from the WHO 3rd International Standard for prolactin, 84/500, used by current assays. A prolactin of 1,800 mIU/L is 84.9 µg/L. µg/L and ng/mL are the same number.
What is the hook effect in prolactinoma?
With a very large prolactinoma, prolactin can be so high that it saturates the antibodies of a sandwich immunoassay and reads falsely low, sometimes in the range expected from stalk compression. Repeating after a 1:100 dilution reveals the true value. Both the 2011 guideline and the 2023 consensus recommend it when a large tumour has an unexpectedly modest prolactin.
What is macroprolactin?
Prolactin bound to immunoglobulin G. It is measured by most immunoassays but has little biological activity, so it can produce a raised result in someone with no symptoms. Polyethylene glycol precipitation separates it. The 2023 consensus advises checking for it when prolactin is moderately raised, below 200 ng/mL.
Can a pituitary tumour raise prolactin without being a prolactinoma?
Yes. Any mass that compresses the pituitary stalk blocks the dopamine that restrains prolactin — the stalk effect. The 2023 consensus puts this at up to about six times the upper limit, and in one series cited by the 2011 guideline a prolactin above 94 µg/L reliably separated prolactinomas from nonfunctioning adenomas.
Related calculators
References
- Melmed S, Casanueva FF, Hoffman AR, et al. Diagnosis and treatment of hyperprolactinemia: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(2):273–288. — "A prolactin level greater than 500 µg/liter is diagnostic of a macroprolactinoma"; "greater than 250 µg/liter usually indicates the presence of a prolactinoma"; risperidone and metoclopramide "may cause prolactin elevations above 200 µg/liter"; medication-induced levels "usually … ranging from 25 to 100 µg/liter"; in 226 nonfunctioning macroadenomas "a prolactin level greater than 94 µg/liter reliably distinguished" prolactinomas; repeat after 1:100 dilution when values are not as high as expected (hook effect); macroprolactin screening in asymptomatic hyperprolactinaemia; prolactin rises about 10-fold in pregnancy, to 150–300 µg/L by term.
- Petersenn S, Fleseriu M, Casanueva FF, et al. Diagnosis and management of prolactin-secreting pituitary adenomas: a Pituitary Society international Consensus Statement. Nat Rev Endocrinol. 2023;19(12):722–740. doi:10.1038/s41574-023-00886-5. — prolactin below five times the upper limit of normal should be retested (strong); stalk compression can raise prolactin up to six times the upper limit; macroprolactin evaluated when prolactin is moderately raised (below 200 ng/mL); 1:100 dilution with a giant adenoma and a normal or slightly raised prolactin.
- Yadav P, Hamrahian AH, Salvatori R. The hook effect in macroprolactinomas: tumor size thresholds, prolactin patterns, and clinical consequences — a systematic review. Pituitary. 2026;29:98. doi:10.1007/s11102-026-01705-6. — smallest confirmed hook-effect tumour 22 mm; 39.1% below 40 mm; median undiluted prolactin 100 ng/mL.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/