How to Convert Prolactin ng/mL to mIU/L
Prolactin is reported in ng/mL (or µg/L) in the US and in mIU/L across much of the world. The conversion factor is 21.2, based on the WHO International Standard 84/500.
Prolactin mIU/L = prolactin ng/mL × 21.2 Prolactin ng/mL = prolactin mIU/L ÷ 21.2 µg/L and ng/mL are identical for prolactin, so 15 ng/mL is the same as 15 µg/L — only mIU/L needs the factor.
Prolactin ng/mL to mIU/L Chart
| ng/mL (µg/L) | mIU/L | Context |
|---|---|---|
| 5 ng/mL | 106 mIU/L | Normal |
| 15 ng/mL | 318 mIU/L | Normal |
| 20 ng/mL | 424 mIU/L | Upper normal (men) |
| 25 ng/mL | 530 mIU/L | Upper normal (women) |
| 100 ng/mL | 2,120 mIU/L | Marked elevation |
| 250 ng/mL | 5,300 mIU/L | Prolactinoma range |
mIU/L to ng/mL Reverse Chart
| mIU/L | ng/mL |
|---|---|
| 100 mIU/L | 4.7 ng/mL |
| 424 mIU/L | 20 ng/mL |
| 530 mIU/L | 25 ng/mL |
| 1,000 mIU/L | 47 ng/mL |
Prolactin Reference Ranges
| Group | ng/mL | mIU/L |
|---|---|---|
| Men | 2 – 18 ng/mL | 42 – 382 mIU/L |
| Women (non-pregnant) | 2 – 29 ng/mL | 42 – 615 mIU/L |
| High (hyperprolactinemia) | Above ~25 ng/mL | Above ~530 mIU/L |
Ranges vary by assay. Pregnancy and breastfeeding raise prolactin well above these levels as a normal physiological response.
What a High Prolactin Level Suggests (by Level)
Roughly, the height of the elevation points toward the cause — though this is only a guide, and interpretation always includes medications and symptoms:
| Level | Common causes |
|---|---|
| Mild (~25–100 ng/mL) | Medications, hypothyroidism, stress, macroprolactin, small prolactinoma |
| Moderate (100–200 ng/mL) | Medications, microprolactinoma |
| Marked (above ~200–250 ng/mL) | Prolactinoma (often a macroadenoma) becomes likely |
The most common causes of a mildly raised prolactin are medications (especially antipsychotics and some anti-nausea drugs) and an underactive thyroid — both worth checking before imaging.
Macroprolactin: The Common False Elevation
This is the nuance most converters never mention. Macroprolactin is prolactin bound to an antibody (IgG). It is biologically inactive but still detected by many assays, so it can make prolactin look high in someone with no symptoms. About one in five raised prolactin results is due to macroprolactin. That is why, when prolactin is elevated without a clear reason, labs often screen for macroprolactin before any further work-up — it can spare an unnecessary scan.
Worked Example
Suppose a report shows 530 mIU/L and you want ng/mL:
530 ÷ 21.2 = 25 ng/mL 25 ng/mL is at the upper limit for women. Converting the other way, a man's upper limit of 20 ng/mL is about 424 mIU/L.
Frequently Asked Questions
How do you convert prolactin ng/mL to mIU/L?
Multiply ng/mL by 21.2. For example, 15 ng/mL is about 318 mIU/L.
Is µg/L the same as ng/mL?
Yes — they are identical (1:1) for prolactin.
What is a normal prolactin?
Under about 20 ng/mL (men) and 25 ng/mL (women), higher in pregnancy.
What is macroprolactin?
An inactive antibody-bound form that can falsely raise measured prolactin — about 20% of high results.
References
- UnitsLab — Prolactin (PRL) unit conversion (ng/mL ↔ mIU/L).
- Prolactin factor 21.2 (WHO IS 84/500); µg/L = ng/mL (1:1); sex-specific reference ranges and macroprolactin from standard endocrinology references.
Conclusion
Converting prolactin between ng/mL and mIU/L is a ×21.2 (and µg/L = ng/mL). Before treating a "high" result as real, remember the two common traps: medications/thyroid, and macroprolactin. Interpret it with your clinician.