FSH and LH: assessing menopausal status and cycle phases

FSH and LH are two gonadotropic hormones that regulate the menstrual cycle and reflect a woman’s reproductive status. They are measured to assess cycle phase, confirm ovulation, or establish where a woman sits on the menopausal transition spectrum. But a number on a lab report without clinical context tells you very little: a single FSH value neither confirms nor excludes a diagnosis of menopause. Here is what these figures mean and when it makes sense to measure them.

What FSH and LH are

Both hormones are produced by the pituitary gland — a small structure at the base of the brain. They act as a pair, governing follicle maturation and the release of an egg.

Follicle-stimulating hormone (FSH) stimulates follicle growth in the ovaries and, along with it, estradiol production. In the first half of the cycle it rises gradually, driving one dominant follicle to mature through to ovulation. After menopause, when the ovaries stop responding to this signal, FSH remains persistently elevated.

Luteinizing hormone (LH) is responsible for triggering ovulation: the sharp mid-cycle LH surge — often called the “LH peak” — initiates egg release. After ovulation it supports the corpus luteum in producing progesterone.

Together, FSH and LH act as a mirror reflecting the state of ovarian reserve. The less the ovaries respond to their signals, the higher both values climb.

Menstrual cycle phases and reference ranges

FSH and LH values depend substantially on which day of the cycle the test is taken. Comparing a result against a reference range without accounting for cycle phase is a common mistake.

These ranges are indicative and correspond to typical IFCC values. Individual laboratories may report slightly different limits depending on their method and reagents — always compare your result to the reference range printed on your own lab report.

The menopausal transition

Menopause is not a single event but a process that unfolds over several years. The STRAW+10 staging system (Harlow et al., 2012) is built entirely on menstrual bleeding patterns: early perimenopause is defined by cycle-length variability of 7 or more days compared with the usual rhythm; late perimenopause by skipped cycles or amenorrhoea lasting 60 or more days; postmenopause is confirmed by 12 consecutive months without menstruation. STRAW+10 contains no numeric FSH thresholds.

According to NICE NG23, an FSH threshold of ≥ 25 IU/L serves as a marker of late perimenopause and is used in women aged 40–45 when the clinical picture is ambiguous. There is, however, an important exception: for women over 45 with typical symptoms — irregular periods, hot flushes, night sweats, and mood changes — NICE NG23 supports establishing the diagnosis of menopause clinically. In this group, routine hormone testing is not routinely performed, as it does not alter clinical management.

When to measure FSH and LH

For women with a regular cycle, the optimal time to test is days 2–5 from the start of menstruation (early follicular phase). This “basal” window produces the most stable values and makes results from different measurements comparable.

Regarding time of day: there is no strict requirement, but morning testing is standard practice, as gonadotropin secretion follows a diurnal rhythm. For reproducibility, it is preferable to repeat tests at the same time of day.

If the cycle is irregular or absent, a clinician determines the optimal timing on an individual basis. A single measurement is rarely sufficient — assessing the trend requires at least two results taken several weeks apart.

Reading FSH together with estradiol

FSH and estradiol are linked by an inverse relationship: as the ovaries begin producing less estradiol, the pituitary raises FSH in an attempt to push the ovaries to work harder. This is why FSH rises and estradiol falls during perimenopause.

Interpreting FSH without considering estradiol can give an incomplete picture: in early perimenopause, FSH may still be within the normal range while estradiol is already fluctuating. Together the two markers provide more information than either one alone. Estradiol is one of the standard components of menopausal status assessment; for detailed reference ranges across cycle phases and the perimenopause — see the estradiol (E2) article.

Common interpretation pitfalls

Combined hormonal contraception suppresses FSH and LH. Combined oral contraceptives suppress the pituitary — FSH and LH values while taking the pill are artificially low. Measuring these hormones during hormonal contraception to assess menopausal status or ovarian reserve is uninformative. The test should be deferred until after the contraceptive is discontinued.

One number is not a diagnosis. FSH varies naturally from cycle to cycle, especially during perimenopause. A single elevated result does not confirm menopause; a single normal result does not exclude perimenopause. At least two measurements taken in separate cycles provide far more information.

Without cycle context the number is meaningless. An FSH of 15 IU/L in the follicular phase is a different finding from an FSH of 15 IU/L at the ovulatory peak. Always note the day of your cycle when having the test done.

When working through the differential diagnosis of amenorrhoea or irregular cycles, clinicians typically assess several hormones together — including TSH and prolactin — rather than FSH and LH alone.

How HealthLab helps track trends

In perimenopause, FSH is most informative not as an isolated data point but as a trend: is it rising gradually or in jumps, and is there a pattern across cycles? A single measurement is a snapshot; several measurements over time reveal the real picture.

HealthLab automatically recognises FSH, LH, and other hormones from PDF lab reports issued by any laboratory, and builds a trend chart over time. You can see immediately how the value is changing from cycle to cycle — without manual data entry or spreadsheets.

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