Luteinizing Hormone (LH) Unit Converter
Luteinizing Hormone (LH) Unit Converter
IU/L, mIU/mL and mU/mL are the same number for LH, so the conversion is the easy part. The hard part is which reference interval applies — and for LH that depends entirely on where in the cycle the sample was taken.
Luteinizing Hormone (LH) converter
IU/L = mIU/mLLH 6.8 IU/L, follicular phase
Units and reference intervals
1 mIU/mL = 10⁻³ IU ÷ 10⁻³ L = 1 IU/L
- IU/L, mIU/mL, mU/mL
- the same number written three ways. The milli- in mIU/mL cancels against the milli- in the volume, so no arithmetic is involved and no factor can be got wrong
- no molar unit
- LH is a heterodimeric glycoprotein whose carbohydrate content varies with the individual, the cycle phase and the assay’s antibody pair, so it has no single molecular mass. It is standardised in international units of biological potency instead, and a nmol/L figure would be a fiction
- WHO 3rd IS 81/535
- the current International Standard for human pituitary LH, which replaced 80/552. Assays calibrated against different standards are not strictly interchangeable, which is why an LH from one laboratory should not be compared with an LH from another
- why five intervals
- LH has no adult female reference interval. The follicular, midcycle and luteal intervals overlap so little that using one combined range would make every surge look abnormal and every case of ovarian failure look normal
- pulsatility
- LH is released in pulses at roughly 60 to 90 minute intervals under GnRH drive, and the peak-to-trough difference within one hour can exceed the width of the reference interval. A single value is a snapshot
Worked example
LH 6.8 IU/L, follicular phase
6.8 IU/L = 6.8 mIU/mL = 6.8 mU/mL — the three units are identical
Within the follicular interval of 1.9–14.6 IU/L
With a follicular FSH of 5.0 IU/L, the LH:FSH ratio would be 1.36 — below the 2 often quoted in polycystic ovary syndrome
Reference intervals by cycle phase and sex
| Group | LH (IU/L, mIU/mL or mU/mL) | What a value at the top of the range means |
|---|---|---|
| Follicular phase (day 2 to 5) | 1.9 – 14.6 | The phase in which LH is usually measured for a fertility work-up, and the one the LH:FSH ratio assumes |
| Midcycle surge | 12.2 – 118.0 | The ovulatory surge. A single high LH here is the expected physiology, not a finding |
| Luteal phase | 0.7 – 12.9 | Suppressed by progesterone. The lowest of the three female phases |
| Postmenopausal | 5.3 – 65.4 | Loss of negative feedback drives LH up. Values are typically above 40 IU/L once the menopause is established |
| Adult male | 1.3 – 9.6 | Read alongside a 9 am testosterone; the pair separates primary from secondary hypogonadism |
LH and the sex steroid, read together
| LH | Sex steroid | Pattern | Where the problem is |
|---|---|---|---|
| High | Low | Hypergonadotropic hypogonadism | The gonad. Primary ovarian insufficiency, Turner or Klinefelter syndrome, chemotherapy or radiotherapy damage, orchidectomy |
| Low or inappropriately normal | Low | Hypogonadotropic hypogonadism | The hypothalamus or pituitary. Functional causes — low weight, heavy exercise, systemic illness — plus hyperprolactinaemia, opioids, anabolic steroids, Kallmann syndrome, pituitary lesions |
| High or high-normal | Normal or raised | Ovulatory or PCOS pattern | Neither, necessarily. A chance surge, or the raised LH drive seen in polycystic ovary syndrome. Confirm the cycle day before doing anything else |
| Suppressed | Suppressed | Exogenous suppression | Treatment. GnRH agonists, combined hormonal contraception, testosterone replacement and anabolic steroids all switch the axis off |
Why LH needs a phase, not a range
Luteinizing hormone is reported in IU/L, mIU/mL or mU/mL, and those three are the same number: the milli- in the unit cancels against the milli- in the volume. There is nothing to get wrong in the conversion, which is why this page spends its effort on the interval instead. LH is a heterodimeric glycoprotein sharing its alpha subunit with FSH, TSH and hCG, and its carbohydrate content varies enough between individuals that no single molecular mass applies to it. It is standardised in international units of biological potency against a WHO preparation — currently the 3rd International Standard, 81/535 — so a molar value would be meaningless and none is offered.
The interval is the difficult part. Across a normal menstrual cycle LH moves through three distinct ranges, and they barely overlap: a luteal upper limit near 13 IU/L, a follicular upper limit near 15, and a midcycle surge that reaches 118. Collapsing those into one adult female range would make every ovulatory surge look pathological and hide every case of ovarian failure sampled off-peak. So this page asks which phase the sample came from and reads it against that phase alone. If the cycle day is unknown, the result is not interpretable, and the honest next step is to repeat the sample on a known day rather than to guess.
Even with the right phase, a single LH is a snapshot of a pulsatile signal. GnRH drives LH in pulses roughly every 60 to 90 minutes, and the difference between a peak and a trough within one hour can be wider than the reference interval itself. This is why a lone borderline LH almost never settles a question, and why the value earns its keep only alongside something else: oestradiol or testosterone to localise the lesion, FSH for the ratio, prolactin when the axis is suppressed for no obvious reason.
Two practical cautions. Assays calibrated against different international standards are not strictly interchangeable, so an LH from one laboratory should not be trended against an LH from another. And LH shares its alpha subunit with hCG, so in pregnancy or in a patient with an hCG-secreting tumour some assays read fractionally high; the effect is small, but it is one more reason not to hang a decision on a single borderline gonadotropin.
Frequently asked questions
Is LH in IU/L the same as mIU/mL?
Yes, numerically identical, and mU/mL is the same again. One milli-international-unit per millilitre is one international unit per litre, because the prefix on the unit cancels against the prefix on the volume. No conversion factor is involved.
What is a normal LH level?
There is no single normal. In a menstruating woman the follicular interval runs 1.9 to 14.6 IU/L, the luteal interval 0.7 to 12.9, and the midcycle surge 12.2 to 118.0. Postmenopausal is 5.3 to 65.4 and adult male 1.3 to 9.6. The cycle phase has to be known before the number means anything.
Why is there no nmol/L for LH?
LH is a glycoprotein whose carbohydrate content varies between individuals and across the cycle, so it has no single molecular mass to divide by. It is standardised in international units of biological potency against a WHO reference preparation instead of by mass.
What does a high LH with a low oestradiol mean?
Primary gonadal failure — the pituitary is driving an ovary that cannot respond. Below the age of 40 this raises premature ovarian insufficiency, which should be confirmed on a repeat sample at least four weeks later rather than diagnosed on one result.
Does a single LH tell you whether someone has ovulated?
No. LH is secreted in pulses every 60 to 90 minutes and the surge lasts under 48 hours, so a single serum value can miss it entirely. A mid-luteal progesterone, or serial urinary LH testing, answers the ovulation question far better.
Related calculators
References
- Mayo Clinic Laboratories. Luteinizing Hormone (LH), Serum. Test ID LH, unit code 602752. Reference values.
- National Institute for Biological Standards and Control. Luteinizing Hormone, Human, Pituitary (3rd International Standard), NIBSC code 81/535.
- Webber L, Davies M, Anderson R, et al. ESHRE guideline: management of women with premature ovarian insufficiency. Hum Reprod. 2016;31(5):926–937.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.