When a woman in her 40s goes to her GP with symptoms that might be perimenopause, the most common test offered is FSH: follicle-stimulating hormone. A high result is taken as confirmation. A normal result is sometimes used to dismiss the possibility entirely. Both interpretations can be wrong.

What FSH and LH Do

FSH (follicle-stimulating hormone) and LH (luteinising hormone) are produced by the pituitary gland at the base of the brain. Both are gonadotropins, hormones that signal the ovaries to do their work.

FSH stimulates the growth and maturation of follicles in the ovary. Each follicle contains a developing egg. As follicles mature, they produce estrogen. LH triggers ovulation: the mid-cycle LH surge causes a mature follicle to release its egg.

The pituitary and ovaries operate as a feedback loop. When estrogen is adequate, the pituitary reduces FSH output. When estrogen is low, the pituitary increases FSH to drive the ovaries harder, the same principle as a thermostat turning up when the room cools.

Why FSH Rises in Perimenopause

As viable follicles decline with age, the ovaries become less responsive to FSH, so the pituitary compensates by producing more, but the ovaries cannot fully respond, and estrogen output becomes more variable and eventually lower.

By the time a woman reaches menopause (defined as 12 consecutive months without a period), FSH is chronically elevated (typically above 25–30 IU/L using most laboratory assays) and estrogen is persistently low. This is the state in which a single FSH measurement is most reliable as a diagnostic indicator.

Why a Single FSH Test Fails in Perimenopause

In perimenopause, the hormonal picture is not stable. FSH rises and falls, sometimes dramatically, from one week to the next, because the ovaries are still responding erratically rather than consistently failing. In a cycle with healthy ovulation, FSH will be low in the follicular phase. In a cycle with impaired ovulation, it will be elevated. In an anovulatory cycle, FSH may be high and estrogen low.

A single blood test captures a single moment in this fluctuating landscape. A woman with classic perimenopausal symptoms could have a completely normal FSH on the day of testing if she happened to be in a cycle with adequate ovarian response. Her test returns as normal, her symptoms are dismissed, and she is no closer to understanding what is happening.

NICE guideline NG23 explicitly states that FSH measurement alone is not reliable for diagnosing perimenopause in women aged 45 and over.[1] In this age group, the clinical picture (symptoms, menstrual irregularity, age) is the primary diagnostic basis. A test is more useful in women under 40 where premature ovarian insufficiency (POI) needs to be ruled out, or in women who have had a hysterectomy and lack cycle irregularity as a guide.

Reading FSH Reference Ranges

Laboratory reference ranges vary by lab and by cycle phase. A result labelled "normal" can mean different things depending on when in the cycle blood was drawn.

FSH reference ranges by phase (approximate): - Follicular phase (days 2–5): 3–10 IU/L - Ovulatory peak: 4–22 IU/L - Luteal phase: 1–9 IU/L - Postmenopause: typically above 25–30 IU/L using most laboratory assays

A result of 12 IU/L returned as "normal" in a woman with night sweats, cycle irregularity, and mood changes tells you very little. Whether 12 is high or low depends entirely on where in the cycle blood was drawn; in perimenopause the cycle itself is irregular.

What LH Adds

LH is rarely tested in isolation for perimenopause, but it appears alongside FSH on many hormone panels. In postmenopause, both FSH and LH are chronically elevated. In perimenopause, LH is typically less reliably elevated than FSH, because the mid-cycle LH surge can still occur even in cycles where follicular development is compromised, meaning a single LH measurement is at least as prone to misinterpretation as FSH, and often more so.

An elevated LH:FSH ratio (above 2:1) with normal or mildly elevated FSH can indicate polycystic ovary syndrome (PCOS) rather than perimenopause. PCOS can persist into the perimenopausal years and produce overlapping symptoms. If this pattern appears, further investigation is warranted before attributing symptoms to perimenopause alone.

Estradiol: Often More Informative

Serum estradiol (E2) measures the circulating level of the primary estrogen produced by the ovaries. In perimenopause, estradiol fluctuates widely, sometimes reaching high levels in the follicular phase of a cycle with aggressive FSH stimulation, then falling sharply. That variability is characteristic of perimenopause and explains much of the cycle-to-cycle unpredictability women experience.

A very low estradiol (below 100 pmol/L or approximately 27 pg/mL, an approximate threshold; specific guideline values vary by assay and clinical context) on a day-2 blood test in a woman with perimenopausal symptoms is consistent with ovarian insufficiency. A high estradiol does not rule out perimenopause; it may represent a surge phase before a cycle in which estrogen subsequently crashes.

Testing both FSH and estradiol on day 2 or 3 of a cycle provides more context than either test alone.

AMH and Ovarian Reserve

Produced by small ovarian follicles, Anti-Mullerian hormone (AMH) reflects ovarian reserve, the number of follicles remaining. It declines steadily with age and is used in fertility medicine to estimate the remaining egg supply.

AMH is not a standard diagnostic test for perimenopause and is not recommended for this purpose by major guideline bodies.[2] Some women have it tested privately. A low AMH indicates diminished ovarian reserve, consistent with perimenopause, but it does not predict symptom severity or timing of menopause at the individual level.

What to Ask Your GP

NICE NG23 supports offering HRT on the basis of clinical assessment in women aged 45 and over, without requiring a blood test to confirm the diagnosis.[1]

If you do want a blood test, or your GP recommends one, the following asks will give the result more diagnostic weight: - Ask for FSH and estradiol together, ideally on day 2 or 3 of a cycle - Ask what cycle phase the sample represents so you can interpret the result correctly - Do not accept a single normal FSH as evidence against perimenopause if your symptoms and age are consistent with it - Repeat testing is more informative than a single snapshot, especially if the first result is ambiguous

If you are under 40 and experiencing these symptoms, ask your GP for a referral to a specialist, as premature ovarian insufficiency (POI) requires a different assessment pathway and should not be assumed to be perimenopause on clinical grounds alone.

A hormone test can support a clinical conversation but cannot replace it. In women aged 45 and over, symptoms plus age are a sufficient basis for a perimenopause diagnosis and a trial of treatment.

References

[1] National Institute for Health and Care Excellence. (2015, updated 2019). Menopause: Diagnosis and management (NICE guideline NG23). NICE. https://www.nice.org.uk/guidance/ng23

[2] Harlow, S. D., Gass, M., Hall, J. E., et al. (2012). Executive summary of the Stages of Reproductive Aging Workshop +10: Addressing the unfinished agenda of staging reproductive aging. Journal of Clinical Endocrinology and Metabolism, 97(4), 1159–1168. https://doi.org/10.1210/jc.2011-3362