Why does my doctor want FSH, LH and estradiol tests on day 2 or 3 of my period?

Early in the cycle, roughly days 2 to 5, hormone levels are at a low, stable baseline, which makes the results easier to compare and interpret. Testing then gives a snapshot of how the brain and ovaries are communicating before a follicle starts to dominate. FSH (follicle stimulating hormone) is released by the pituitary gland to stimulate follicles. A raised early-cycle FSH can suggest the ovaries need more 'push', which is linked with lower ovarian reserve. LH (luteinising hormone) is often compared with FSH; a higher LH relative to FSH is sometimes seen in PCOS, though it isn't used alone to diagnose it. Estradiol is measured alongside FSH because a high early estradiol can suppress FSH and make it look falsely normal. These tests are often combined with AMH and an antral follicle count scan to build a fuller picture of ovarian reserve. AMH can be tested on any day, which is why it doesn't need the same timing. Thyroid and prolactin can usually be tested any day too. If your period starts on a weekend or holiday, ask the lab or doctor whether day 3 to 5 is acceptable. Results vary between cycles, so your doctor will interpret them along with your age and other tests rather than on one number.