Prolactin Unit Converter
Prolactin Unit Converter
Convert prolactin between µg/L, ng/mL and mIU/L at 21.2 mIU/L per µg/L — then check for macroprolactin and the hook effect, which change what the number means.
Prolactin converter
Mass ⇄ IUProlactin 12 µg/L, non-pregnant female
Formula and conversion factors
µg/L = mIU/L ÷ 21.2
ng/mL = µg/L (numerically identical)
- 21.2
- the calibration of the WHO 3rd International Standard for prolactin, 84/500 — a potency assignment, not a molecular mass, so it can change when the standard changes
- ng/mL
- numerically identical to µg/L; one microgram per litre is one nanogram per millilitre
- no molar unit
- prolactin is standardised in international units against a WHO preparation and circulates as monomeric, dimeric and macromolecular forms, so a single molar value would be meaningless
- older standards
- results reported against earlier standards used different factors, so a mIU/L figure from an old report cannot be converted with confidence
Worked example
Prolactin 12 µg/L, non-pregnant female
12 µg/L = 12.0 ng/mL — the two units are numerically identical
12 × 21.2 = 254 mIU/L against the WHO 3rd International Standard 84/500
Within the non-pregnant female interval of 4.0–23.0 µg/L
Prolactin across the three units
| µg/L | ng/mL | mIU/L |
|---|---|---|
| 4.0 — typical lower limit | 4.0 | 85 |
| 12.0 | 12.0 | 254 |
| 15.0 — typical male upper limit | 15.0 | 318 |
| 23.0 — typical female upper limit | 23.0 | 488 |
| 50.0 | 50.0 | 1,060 |
| 100.0 | 100.0 | 2,120 |
| 250.0 — typical of a macroprolactinoma | 250.0 | 5,300 |
Two assay problems that change what to do
| Problem | What happens | Who it affects | What to ask for |
|---|---|---|---|
| Macroprolactin | Prolactin bound to IgG forms a large complex that the assay detects but that is not biologically active, so the reported prolactin is raised in a well person | Typically an asymptomatic patient with a moderately raised prolactin and no galactorrhoea, amenorrhoea or visual symptoms | Macroprolactin screening, usually by polyethylene glycol precipitation, before any pituitary imaging |
| Hook effect | A very high prolactin saturates both antibodies of the sandwich assay, the sandwich fails to form, and the reported value is falsely normal or low | A patient with a large pituitary mass and symptoms of a prolactinoma whose prolactin comes back unremarkable | A repeat assay on a diluted sample, typically 1 in 100, before concluding the mass is non-functioning |
A calibration, not a molecular mass — and two assay traps
Prolactin is reported in µg/L or ng/mL, which are numerically identical, and in mIU/L. The factor between them is 21.2: one microgram per litre is 21.2 milli-international units per litre, so a prolactin of 12 µg/L is 254 mIU/L. That number is not a molecular mass. It is the potency assigned to the WHO 3rd International Standard for prolactin, 84/500, against which modern assays are calibrated, and it is why figures reported against earlier standards used different factors and cannot be reconverted with confidence.
No molar unit is offered. Prolactin circulates as a monomer, a dimer and as large macromolecular complexes, and it is standardised by bioactivity against a WHO preparation rather than by mass, so a single pmol/L value would be meaningless. Before interpreting any raised result, the ordinary explanations should be excluded first: venepuncture stress, pregnancy, breastfeeding, nipple stimulation, primary hypothyroidism, chronic kidney disease and, above all, drugs — antipsychotics, metoclopramide, domperidone, some antidepressants and oestrogens.
Then come two analytical problems that change what a reader should do with the number. The first is macroprolactin: prolactin bound to immunoglobulin forms a large complex that the assay detects but that is not biologically active. It is a common cause of an apparently raised prolactin in a person with no symptoms at all, and it accounts for a substantial minority of hyperprolactinaemia reports. Where the prolactin is moderately raised but there is no galactorrhoea, no menstrual disturbance and no visual field defect, macroprolactin screening by polyethylene glycol precipitation should be requested before any pituitary imaging.
The second runs the other way and is the more dangerous. In a very large prolactinoma the prolactin concentration can be so high that it saturates both the capture and the detection antibody of the sandwich assay, the sandwich never forms, and the reported result comes back falsely normal or even low. This hook effect has led to macroprolactinomas being labelled non-functioning adenomas and operated on rather than treated medically. When a patient has a large pituitary mass and an unremarkable prolactin, ask the laboratory to repeat the assay on a diluted sample.
Frequently asked questions
How do I convert prolactin from µg/L to mIU/L?
Multiply by 21.2, the calibration of the WHO 3rd International Standard for prolactin, 84/500. A prolactin of 12 µg/L is 254 mIU/L. To go the other way, divide the mIU/L figure by 21.2.
Is ng/mL the same as µg/L for prolactin?
Yes, exactly. One microgram per litre is one nanogram per millilitre, so the two are numerically identical and no conversion is needed between them.
Why is 21.2 not a molecular mass?
Prolactin is standardised by bioactivity in international units against a WHO reference preparation rather than by mass, and it circulates as monomeric, dimeric and macromolecular forms. The figure of 21.2 is the potency assigned to the 3rd International Standard, 84/500, so it is a calibration that changes when the standard changes. Values reported against earlier standards used different factors and cannot be reconverted reliably.
What is macroprolactin and when should it be tested for?
Macroprolactin is prolactin bound to immunoglobulin, forming a large complex that immunoassays detect but which has little biological activity. It is a common cause of an apparently raised prolactin in someone with no symptoms, and it should be screened for by polyethylene glycol precipitation whenever a moderately raised prolactin is not accompanied by galactorrhoea, menstrual disturbance or visual symptoms. Doing so before imaging avoids a great deal of unnecessary investigation.
Can a very high prolactin be reported as normal?
Yes, and this is the hook effect. At extremely high concentrations prolactin saturates both antibodies of a sandwich immunoassay so the sandwich cannot form, and the reported value comes back falsely normal or low. When a patient has a large pituitary mass and an unremarkable prolactin, the assay should be repeated on a diluted sample before the mass is called non-functioning.
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.
- Fahie-Wilson M, Smith TP. Determination of prolactin: the macroprolactin problem. Best Pract Res Clin Endocrinol Metab. 2013;27(5):725–742.
- St-Jean E, Blain F, Comtois R. High prolactin levels may be missed by immunoradiometric assay in patients with macroprolactinomas. Clin Endocrinol (Oxf). 1996;44(3):305–309.