LH to FSH Ratio Calculator
LH to FSH Ratio Calculator
Calculate the LH to FSH ratio and see why a value above 2 is a classic feature of PCOS but not, on its own, a diagnostic one.
LH to FSH Ratio
Ratio, timing-dependentLH 12.5 IU/L, FSH 5.0 IU/L
Formula
- LH, FSH
- IU/L, from the same sample
- sample timing
- early follicular phase, cycle days 2 to 5 — the ratio is meaningless outside this window
Worked example
LH 12.5 IU/L, FSH 5.0 IU/L
12.5 ÷ 5.0 = 2.50
Reading the ratio in context
| Pattern | Consider |
|---|---|
| LH:FSH > 2, early follicular | Supportive of PCOS, but present in only about 50–60% of cases |
| FSH high relative to LH | Diminished ovarian reserve or primary ovarian insufficiency |
| Both LH and FSH low | Hypogonadotropic hypogonadism — weight loss, excessive exercise, stress, or a pituitary cause |
| Sample taken mid-cycle | LH surge invalidates the ratio even in a normal cycle |
A classic finding, not a diagnostic one
An LH to FSH ratio above 2 is a classic feature of polycystic ovary syndrome, reflecting the relatively increased LH pulsatility seen in the condition. It is not, however, a diagnostic feature — it is present in only about 50 to 60% of women with PCOS, and it was removed from the Rotterdam diagnostic criteria for that reason. A normal ratio does not exclude PCOS, and a raised one does not confirm it in isolation.
Timing matters more than almost anything else when interpreting this result. Sample in the early follicular phase, cycle days 2 to 5, because the mid-cycle LH surge sends the ratio far above 2 in a completely normal cycle, and a sample taken at the wrong time will manufacture a false-positive pattern. The combined oral contraceptive suppresses both gonadotrophins together and makes the ratio uninterpretable while it is being taken.
The ratio is also useful for what it points to outside PCOS. A high FSH relative to LH points elsewhere entirely — towards diminished ovarian reserve or primary ovarian insufficiency, where FSH rises as ovarian feedback fails. Both gonadotrophins being low together suggests hypogonadotropic hypogonadism, seen with significant weight loss, excessive exercise, psychological stress, or a hypothalamic-pituitary cause, and warrants its own separate work-up rather than being read as a PCOS result.
Frequently asked questions
Does an LH:FSH ratio above 2 confirm PCOS?
No. It is a classic finding but present in only about 50 to 60% of women with PCOS, which is why it was removed from the Rotterdam diagnostic criteria. Diagnosis requires the clinical and ultrasound picture as well.
When should LH and FSH be sampled?
In the early follicular phase, cycle days 2 to 5. A sample taken around ovulation will show a markedly raised ratio due to the LH surge even in a completely normal cycle.
Can the contraceptive pill be used while testing this ratio?
No. The combined oral contraceptive suppresses both LH and FSH, making the ratio meaningless. Testing should be done off it.
What does a high FSH relative to LH suggest?
Diminished ovarian reserve or primary ovarian insufficiency, where FSH rises as ovarian negative feedback weakens — a different picture from PCOS and warranting separate investigation.
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References
- Rotterdam ESHRE/ASRM-Sponsored PCOS Consensus Workshop Group. Revised 2003 consensus on diagnostic criteria and long-term health risks related to PCOS. Fertil Steril. 2004;81(1):19–25.
- Teede HJ et al. Recommendations from the 2023 international evidence-based guideline for the assessment and management of PCOS. Fertil Steril. 2023;120(4):767–93.