Menopause: the supply runs out
A woman is born with all the eggs she will ever have, and the pool shrinks over decades. Menopause is the point where the follicle supply is essentially exhausted — defined, in practice, as 12 months with no period, on average around age 51. With few follicles left, the ovary can no longer make much estradiol or inhibin. Trace it through the loop from guide 4: with estrogen and inhibin gone, the brain loses its negative-feedback signal and pushes harder. So FSH (and LH) climb to high levels. A high FSH with a low estradiol is the classic hormonal fingerprint of menopause.
Low estrogen explains the familiar experience of menopause: hot flushes, sleep disruption, vaginal dryness, and — over the longer term — faster bone loss, because estrogen normally restrains the cells that break down bone. For some women, hormone replacement therapy eases symptoms by replacing estrogen (with progesterone added if the uterus is present, to protect the lining). It is a real benefit-and-risk conversation, best had individually with a clinician.
PCOS: a feedback tangle
Polycystic ovary syndrome (PCOS) is the most common hormonal disorder in women of reproductive age. The egg supply is fine — the problem is regulation. The hallmark is too much androgen plus disordered ovulation. With excess androgens, several follicles start but none becomes a clear dominant one, so ovulation is irregular or absent. That is why PCOS often shows up as long, unpredictable cycles or absent periods, alongside androgen-driven features like acne or extra hair growth, and many small follicles seen on ultrasound.
Where does the extra androgen come from, and why won't it convert to estrogen? Two threads weave together. First, the brain's signaling skews toward LH over FSH, and (recall guide 2) LH drives the theca cells to pump out androgens while the relatively low FSH means less aromatase to convert them onward. Second, and crucially, many women with PCOS have insulin resistance: the high insulin that results both pushes the ovary's theca cells to make even more androgen and lowers SHBG, so more free, active androgen circulates. The result is a self-reinforcing tangle rather than a single broken part.