FSH, LH and oestradiol, explained
FSH and LH are pituitary hormones that tell your ovaries to grow a follicle and release an egg. Oestradiol is the main oestrogen the growing follicle makes in reply. Doctors test the three together, early in the cycle, because each one only makes sense in the light of the other two.
Three hormones run your menstrual cycle between them. Two come from a small gland at the base of your brain; one comes from your ovaries. Understanding how they talk to each other is the key to understanding almost any hormone result you will ever be given.
The cast
- FSH (follicle-stimulating hormone). Made in the pituitary gland. Its job is to tell the ovaries to grow a follicle, the sac that will release this month's egg.
- LH (luteinising hormone). Also from the pituitary. A sharp surge of LH mid-cycle is the trigger that releases the egg: ovulation. You will see it spelt "luteinizing hormone" on overseas sites; it is the same hormone.
- Oestradiol. The main oestrogen, made by the growing follicle in the ovary. It builds the lining of the uterus and, crucially, signals back to the brain about how the ovaries are doing.
How they work together
Think of it as a conversation. Early in your cycle the brain sends FSH to the ovary. The ovary responds by growing a follicle, which makes oestradiol. Rising oestradiol tells the brain "message received, you can ease off the FSH". When oestradiol peaks, the brain fires off the LH surge, the egg is released, and the cycle moves into its second half.
This feedback loop is why doctors never read one of these hormones on its own. A high FSH means something different depending on what oestradiol is doing at the same time.
After ovulation the conversation changes register. The empty follicle becomes the corpus luteum and makes progesterone, which holds the uterine lining in place. FSH and LH fall back to baseline, and if no pregnancy follows, progesterone drops, the lining is shed, and the whole exchange starts again on day one of the next bleed.
Why the day you test matters
That is the window KnowLuna guides you towards for the fertility and hormone panels. If your cycles are irregular or absent, your doctor reads the results knowing that, which changes the interpretation rather than making the test useless. Our short guide to the day 3 hormone test covers how to count day one and what to do if you miss the window.
What the patterns can suggest
Always as a pattern, never as a single number, and always for a doctor to interpret:
- High FSH with low oestradiol early in the cycle can suggest the ovaries are responding less readily, which is seen as ovarian reserve declines and through the menopause transition.
- LH higher than FSH at baseline is a pattern sometimes seen in polycystic ovary syndrome, though it is not present in everyone with PCOS and is not needed for the diagnosis.
- Low FSH and low LH together can point towards the signal from the brain being suppressed, which can happen with significant stress, under-eating, very intensive exercise or some pituitary conditions.
- Everything in range is common, and useful: it helps rule things out and gives you a baseline.
This article is the conceptual explainer. If you have a result in hand and want to know what the numbers mean, the detail lives in two companion articles: what do FSH and LH levels mean, with typical day 2 to 3 ranges, the LH to FSH ratio and what high or low readings can point to; and oestradiol explained, which follows oestradiol across the cycle and into perimenopause.
Why they are tested together for fertility, PCOS and perimenopause
The same three markers answer three different questions, depending on who is asking. In a fertility work-up, the early-cycle FSH and oestradiol pair shows how readily the ovaries are responding, read alongside AMH. In a PCOS assessment, the interest is in the LH to FSH relationship and in ruling out other causes of irregular cycles, alongside the androgens. In perimenopause, the pattern of a rising FSH with a falling oestradiol is what doctors look for, with the large caveat that both swing from month to month during the transition, so one reading rarely settles the question.
That is why the same trio appears in the fertility panel, the hormone panel and the perimenopause panel, each time with different companions and a different question in mind.
Things that change the picture
- Hormonal contraception suppresses this whole loop by design. Baseline readings on the pill mostly reflect the pill, not you. Our guide to contraception and hormone tests covers each method.
- Pregnancy and breastfeeding change all three.
- Perimenopause makes them erratic from month to month, which is exactly why a single result cannot confirm or rule out menopause.
- Raised prolactin or a thyroid problem can quieten the loop from upstream, which is why both are usually checked alongside.
The takeaway
FSH, LH and oestradiol are a set, not three separate tests. Read together, at the right time, by someone who knows your history, they tell a coherent story about your cycle. Read alone, off a printout, they mostly cause worry. That gap is the reason every KnowLuna result comes with a doctor.
Common questions
What do LH and FSH levels tell you?
Together they show how the brain and ovaries are communicating. A low, quiet baseline early in the cycle suggests the ovaries are responding readily. A high FSH can point to ovaries needing more stimulation, as in declining ovarian reserve or the menopause transition. Low levels of both can suggest the signal from the brain is suppressed. Your doctor reads them with oestradiol and your history.
What is the LH:FSH ratio?
It is simply LH divided by FSH on an early-cycle sample. In most women the two sit at similar levels, so the ratio is around one. A ratio where LH is clearly higher than FSH is a pattern sometimes seen in PCOS. It is a supporting clue, not a diagnostic test: many women with PCOS have a normal ratio, and it is not part of the diagnostic criteria.
Why is oestradiol tested with FSH?
Because oestradiol controls FSH. As a follicle grows and makes more oestradiol, the brain lowers FSH in response. A normal-looking FSH next to a high oestradiol may be a high FSH that is being masked, while a high FSH next to a low oestradiol tells a clearer story. Without oestradiol, FSH is a number with half its context missing.
What happens when LH is low?
A low LH on its own is usually not meaningful, because LH is naturally low for most of the cycle and only surges around ovulation. Low LH together with low FSH can point to the signal from the brain being quietened, which can happen with significant stress, low body weight, heavy training, hormonal contraception or, less commonly, a pituitary problem. Your doctor looks at the pattern, not the single value.
What is the difference between FSH and TSH?
They are unrelated hormones that happen to share a letter. FSH is follicle-stimulating hormone, from the pituitary to the ovaries, and drives your cycle. TSH is thyroid-stimulating hormone, from the pituitary to the thyroid, and sets your metabolic pace. Both are often on the same panel because thyroid problems can disturb cycles and mimic hormonal symptoms.
Sources and further reading
- Pathology Tests Explained (RCPA): follicle stimulating hormone (FSH)
- Pathology Tests Explained (RCPA): luteinising hormone (LH)
- Pathology Tests Explained (RCPA): oestradiol
- Healthdirect: follicle stimulating hormone (FSH)
- Healthdirect: oestrogen blood test
- Better Health Channel: menstrual cycle
General information only, not medical advice. If your symptoms are severe or sudden, see a doctor promptly. This page does not replace a consultation with a doctor who knows your history; read our medical disclaimer.