FSH Unit Converter

mIU/mL, IU/L and mU/mL are the same number for FSH — this page confirms that and then does the part that matters, the reference intervals.

Last updated 4 Sep 2026

FSH converter

IU/L = mIU/mL

FSH

mIU/mL, IU/L and mU/mL are numerically identical — the value does not change.

Reference groupChoose the group that matches the sample. Ovarian reserve assessment uses a day 2 to 5 sample, read against the follicular interval.

6.50IU/LExample

FSH 6.5 mIU/mL, follicular phase

Units and what they mean

IU/L = mIU/mL × 1
mU/mL = mIU/mL × 1
All three units are numerically identical

× 1: one milli-international unit per millilitre is one international unit per litre; the conversion is a relabelling and the number never changes
no molar unit: FSH is standardised in international units against a WHO reference preparation, so no molar conversion is meaningful and none is offered
WHO standard: results are traceable to an international standard, but assays are not fully harmonised, so values from different laboratories are not strictly comparable
day 2 to 5: the conventional timing for an early follicular FSH used in ovarian reserve assessment

Worked example

FSH 6.5 mIU/mL, follicular phase
6.5 mIU/mL = 6.50 IU/L — the units are identical, so the figure is unchanged
= 6.50 mU/mL
Within the follicular phase interval of 3.5–12.5 mIU/mL

FSH reference intervals (identical in mIU/mL, IU/L and mU/mL)

One column serves all three units because they are the same number. Intervals are representative and assay-dependent; use the issuing laboratory's own.
	FSH
Follicular phase	3.5 – 12.5
Mid-cycle peak	4.7 – 21.5
Luteal phase	1.7 – 7.7
Postmenopausal	25.8 – 134.8
Adult male	1.5 – 12.4

FSH read with oestradiol

The gonadotrophin only becomes informative when it is read with the gonadal steroid; either value alone cannot localise the problem.
FSH	Oestradiol	Interpretation	Consider
Raised	Low	Primary gonadal failure	Menopause; premature ovarian insufficiency below age 40; Turner syndrome; previous chemotherapy or pelvic radiotherapy
Low or inappropriately normal	Low	Hypogonadotrophic hypogonadism	Functional hypothalamic amenorrhoea, hyperprolactinaemia, pituitary disease, significant weight loss or exercise
Normal	Normal	No gonadotrophin abnormality demonstrated	Does not exclude diminished ovarian reserve — anti-Müllerian hormone and antral follicle count are more sensitive
Normal or low with a raised LH	Normal	Raised LH:FSH ratio	Supportive of polycystic ovary syndrome but not diagnostic on its own

A relabelling, and the interpretation that actually matters

There is no conversion to do here. FSH is reported in mIU/mL, IU/L and mU/mL, and all three are numerically identical: one milli-international unit per millilitre is one international unit per litre. An FSH of 6.5 mIU/mL is 6.5 IU/L and 6.5 mU/mL. This page exists because the three labels look different enough to worry people, and because the useful content is not the arithmetic but the reference intervals and what the value means alongside oestradiol.

No molar unit is offered, and that is deliberate. FSH is a heterodimeric glycoprotein whose glycoforms differ in mass and biological activity, so it is standardised by bioactivity in international units against a WHO reference preparation rather than by mass. A pmol/L figure would have no defensible value. Traceability to that standard is not the same as harmonisation, however: different manufacturers’ assays still give different numbers on the same sample, so a result should be read against the issuing laboratory’s interval and serial values compared only within one laboratory.

The interpretation depends entirely on reading FSH with oestradiol. A raised FSH with a low oestradiol indicates primary gonadal failure, the ovary having stopped responding and the pituitary compensating; below the age of 40 that pattern raises premature ovarian insufficiency, which should be confirmed on a repeat sample at least four weeks later before it is discussed as a diagnosis. A low or inappropriately normal FSH with a low oestradiol points instead to the hypothalamus or pituitary, and prompts prolactin, the other pituitary hormones and a review of weight, exercise and stress.

For ovarian reserve, the conventional test is an early follicular FSH taken on day 2 to 5 of the cycle. A raised value suggests diminished reserve, but the test is insensitive and a normal value does not exclude it, because FSH only rises once follicular decline is well advanced. Anti-Müllerian hormone and the antral follicle count detect the change earlier. None of these tests predicts natural fertility well enough to be used to counsel a woman against trying to conceive.

Frequently asked questions

Is mIU/mL the same as IU/L for FSH?

Yes, exactly. One milli-international unit per millilitre equals one international unit per litre, and mU/mL is the same again. An FSH of 6.5 mIU/mL is 6.5 IU/L, and no calculation is required.

Why is there no pmol/L for FSH?

FSH is a glycoprotein whose glycoforms differ in mass and bioactivity, so it is standardised by bioactivity in international units against a WHO reference preparation rather than by mass. A molar concentration would have no meaningful single value.

What does a raised FSH with a low oestradiol mean?

It indicates primary gonadal failure: the ovary is not responding and the pituitary is compensating. Below the age of 40 this raises premature ovarian insufficiency, which should be confirmed on a repeat sample at least four weeks later.

When should FSH be measured for ovarian reserve?

On day 2 to 5 of the cycle, the early follicular phase. A raised value suggests diminished reserve, but a normal value does not exclude it, because FSH rises only once follicular decline is advanced. Anti-Müllerian hormone and antral follicle count are more sensitive.

Can FSH results be compared between laboratories?

Not strictly. Results are traceable to a WHO standard, but the assays are not fully harmonised and different platforms give different numbers on the same sample. Compare serial results within one laboratory.

Related calculators

References

National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. NICE clinical guideline CG156. 2013, updated 2017.
Webber L, Davies M, Anderson R, et al. ESHRE guideline: management of women with premature ovarian insufficiency. Hum Reprod. 2016;31(5):926–937.
Practice Committee of the American Society for Reproductive Medicine. Testing and interpreting measures of ovarian reserve: a committee opinion. Fertil Steril. 2020;114(6):1151–1157.