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Hormone results in two unit systems
Hormone results are some of the hardest lab values to compare across countries. US laboratories usually report mass units such as ng/dL, pg/mL or µg/dL, while most other countries use molar units such as nmol/L or pmol/L – and a few hormones are measured in international units. Each hormone has its own conversion factor, so a single rule of thumb does not work.
The converters in this section cover:
- [Total testosterone](/testosterone-ng-dl-to-nmol-l) – ng/dL × 0.0347 = nmol/L
- [Free testosterone](/free-testosterone-pg-ml-to-pmol-l) – pg/mL × 3.467 = pmol/L
- [Estradiol](/estradiol-pg-ml-to-pmol-l) – pg/mL × 3.671 = pmol/L
- [Progesterone](/progesterone-ng-ml-to-nmol-l) – ng/mL × 3.18 = nmol/L
- [Free T4](/free-t4-ng-dl-to-pmol-l) – ng/dL × 12.87 = pmol/L
- [Free T3](/free-t3-pg-ml-to-pmol-l) – pg/mL × 1.536 = pmol/L
- [Cortisol](/cortisol-mcg-dl-to-nmol-l) – µg/dL × 27.59 = nmol/L
- [Prolactin](/prolactin-ng-ml-to-miu-l) – ng/mL × 21.2 = mIU/L
- [DHEA-S](/dhea-s-mcg-dl-to-umol-l) – µg/dL × 0.02714 = µmol/L
Quick reference examples
| Hormone | US value | SI value |
|---|---|---|
| Total testosterone | 300 ng/dL | 10.4 nmol/L |
| Free testosterone | 50 pg/mL | 173 pmol/L |
| Estradiol | 100 pg/mL | 367 pmol/L |
| Progesterone | 10 ng/mL | 31.8 nmol/L |
| Free T4 | 1.2 ng/dL | 15.4 pmol/L |
| Free T3 | 3.0 pg/mL | 4.6 pmol/L |
| Cortisol (morning) | 15 µg/dL | 414 nmol/L |
| Prolactin | 20 ng/mL | 424 mIU/L |
| DHEA-S | 200 µg/dL | 5.4 µmol/L |
Why hormone results are tricky
Timing matters
Many hormones follow daily or monthly rhythms. Testosterone and cortisol are highest in the early morning, so tests are usually taken between about 7 and 10 am. Estradiol and progesterone vary across the menstrual cycle – progesterone is typically measured about seven days before an expected period to confirm ovulation. Prolactin rises with stress, sleep and breast stimulation.
Assays differ
Hormones are often measured with immunoassays, and results can vary between manufacturers. Reference ranges should always come from the laboratory that ran the test. For testosterone, the US CDC runs a standardisation programme to improve consistency, and mass spectrometry is preferred for low levels, such as testosterone in women and children or estradiol in men.
Interferences
High-dose biotin supplements can distort many hormone immunoassays, including thyroid tests. Macroprolactin – a large, inactive complex – can make prolactin look high. Heterophile antibodies can cause unpredictable errors. If a result does not fit the clinical picture, these are worth considering.
Total versus free hormones
Most testosterone, estradiol, cortisol and thyroid hormone in blood is bound to carrier proteins. Only the small free fraction is active. Changes in binding proteins – for example, SHBG rising with age, oral oestrogen or hyperthyroidism, and falling with obesity and insulin resistance – change total levels without necessarily changing free levels. That is why free testosterone and free T4 are often measured or calculated.
When to seek medical advice
Hormone results should be interpreted by a clinician alongside symptoms and other tests. Seek advice for symptoms such as unexplained fatigue, weight change, menstrual changes, infertility, low libido, erectile dysfunction, breast discharge, or signs of thyroid disease. Very low morning cortisol with dizziness, vomiting or collapse can indicate adrenal insufficiency and needs urgent assessment.
Frequently asked questions
Why are my hormone results different from a friend's lab in another country?
Different units, assays and reference ranges. Convert the units, then compare with the range on each report.
Does the time of my blood test matter?
Yes, especially for testosterone and cortisol (morning tests), and estradiol and progesterone (cycle day).
Should I stop biotin before hormone tests?
Ask your clinician. Stopping high-dose biotin for at least two days before testing is often advised.