What actually happens to your hormones?

Why does perimenopause happen at all?

You're born with all the egg-containing follicles you'll ever have. Over decades they run down. Fewer follicles means less of two hormones they make — AMH and inhibin B. Inhibin B normally helps keep FSH (follicle-stimulating hormone) in check, so as it drops, the pituitary turns FSH up, trying to push the remaining follicles to respond.

Isn't estrogen just low in perimenopause?

No — this is the most common misunderstanding. In a study tracking hormones across the stages (Hale et al., 2007), AMH fell 10- to 15-fold and inhibin B became largely undetectable, while FSH rose — and in cycles where ovulation still happened, estradiol rose with each stage rather than falling. What changes is stability. High FSH is thought to drive bursts of estrogen, followed by drops, and those swings — not a steady decline — are thought to explain a lot of the unpredictability: heavy periods, breast tenderness, mood shifts, and symptoms that come and go.

What happens to progesterone?

Progesterone is made after ovulation. In that same study, progesterone after ovulation fell across the stages, and cycles without ovulation became more frequent in late perimenopause. So progesterone tends to drop before estrogen does.

What about testosterone?

Testosterone in women declines across adult life — mostly with age, and mostly in the earlier reproductive years. In a large Australian study, natural menopause itself had no separate effect on testosterone. Surgical removal of both ovaries is different: women who'd had it had lower testosterone.

Why do hot flushes happen?

Hot flushes and night sweats start in the hypothalamus — the brain's thermostat. A group of neurons called KNDy neurons is normally kept in check by estrogen. When estrogen falls, they become overactive and signal through neurokinin B and its receptor, NK3. The result is a body that tolerates a narrower range of temperatures, so a small rise in core temperature can trigger flushing and sweating to dump heat. This is the target of two newer non-hormonal medicines: fezolinetant blocks the NK3 receptor, and elinzanetant blocks both NK1 and NK3.

What changes in the rest of the body?

Estrogen receptors are everywhere, so the effects are too:

• Bone: in the SWAN study, bone density loss sped up in late perimenopause and was fastest in the first years after the final period, at both the spine and the hip.
• Heart and blood vessels: LDL cholesterol and apolipoprotein B rise from the year before to the year after the final period, beyond what aging alone explains, and fat shifts toward the abdomen (American Heart Association, 2020).
• Brain: in SWAN, the usual improvement from practice on memory and processing-speed tests stalled during perimenopause, then performance rebounded after menopause.
• Vagina, vulva and bladder: tissue gets thinner and drier — the genitourinary syndrome of menopause (GSM).