What is FSH?

When too little FSH is reaching the ovary, the fault can lie in the pituitary that releases it or in the brain that times the release, and the FSH figure on its own does not say which.

LH, luteinising hormone, is released in the same pulses and acts on the same follicle. Reading them together shows whether the pituitary signal as a whole has changed or only the FSH side of it. Oestradiol is made by the follicle as it grows, so it registers what the ovary has done with the FSH and LH signal rather than the signal itself.

How often the brain signals the pituitary, and how strongly, sets how much FSH it releases. A growing follicle releases oestradiol, and that oestradiol reaches the pituitary and lowers the FSH it sends next.

In those who have a menstrual cycle, FSH rises and falls across the month as a follicle develops. A single sample lands at one point on that curve, and a sample a year later can land anywhere else on it, so neither traces the cycle. What a year's gap can show is a change in pituitary output too large for cycle timing to explain. This is Do Health's monitoring schedule, not a general recommendation for everyone.