FSH tests

High levels of follicle-stimulating hormone (FSH) can be a sign that the ovaries are not working as well as they should be. FSH is produced by the pituitary, a gland at the base of the brain. As the name suggests, FSH stimulates the follicles, which enclose the eggs, to develop. FSH works in a feedback loop with estrogen: As the follicle develops, it makes estrogen, which then travels back to the pituitary, which lowers FSH production in response. This loop repeats every month and helps regulate the menstrual cycle.

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As the ovaries become depleted of eggs, the pituitary must produce a higher level of FSH before it gets a response. While this is normal in a woman approaching menopause, some women may have ovaries that act older than they should. For example, if a 35-year-old woman has FSH levels like those of a 45-year-old, her chances for conception may be like those of an older women. Women with low ovarian reserve (indicated by high levels of FSH) usually do not respond well to fertility medications.

Because FSH goes up and down over the course of a month, it has to be measured at a set time in the cycle. Typically FSH levels are measured with a blood test between days two and four of the menstrual cycle (day one is the first day of menstrual bleeding). FSH can vary from month to month; your doctor will use the highest measurement. You don't need to fast before an FSH blood test. The higher the FSH value, the lower the likelihood of pregnancy, because it suggests your ovaries aren't as responsive as they should be.

Another test of FSH is the so-called clomid challenge, named after a brand of fertility drug. For this test, a woman takes clomiphene citrate, a drug that stimulates the ovaries, from days five to nine of her menstrual cycle. On day 10, she has blood taken for a second FSH test. As with the day three FSH test, a higher level indicates the ovaries may not be functioning well. Some women with a normal day three FSH test have high FSH with the clomid challenge test.