Follicle-stimulating hormone (FSH) plays an important role in the reproductive lives of both men and women. In women, FSH levels vary throughout their menstrual cycles, whereas in men, FSH levels remain fairly constant.

Women's reproductive cycles are usually far more complicated than men's, so it's probably easiest to start with the role of FSH in women. Women are born with a certain number of eggs (ova) in their ovaries at birth. A good number of those ova are absorbed by the body before puberty. After puberty, once the woman's menstrual cycles begin, the woman becomes fertile. Her cycle begins on the first day of her period, and lasts on average around 28 days. About halfway through the cycle (usually between day 11 and 14), the woman experiences a surge of FSH in her ovaries. This stimulates her ovaries to bring forward an immature egg inside a follicle. The egg matures, and then the follicle releases the egg into the fallopian tubes. After this, FSH levels drop as the woman's body prepares for a potential pregnancy.

In men, FSH is responsible for sperm production in the testicles. For either a man or a woman, FSH levels that are too high or too low may result in reduced fertility, and that may require hormone therapy, or even more invasive therapy, like in-vitro fertilization.

If a couple has tried to conceive for over a year with no success, their doctor may want to test their levels of FSH, among other things.

For a man, it is just a simple blood test. If his FSH levels are too high, it could mean that his testicles aren't functioning properly and he is not producing enough sperm. It could also mean that the testicles have been damaged by chemotherapy or by an overuse of alcohol or drugs. FSH levels that are too low mean that the man may be experiencing too much stress or that his brain is not releasing hormones properly, leading to lack of sperm production.

An FSH test for a woman is more complicated. She will likely have a baseline FSH test done during her period, when her FSH levels should be low (since her ovaries are not maturing an egg during this time). Then around the time of her ovulation, another test is done, and that FSH level is compared to her baseline level.

If her FSH levels are too low, she is likely experiencing a lot of stress, to the point where it influences her hormonal balance. She may also be starving or malnourished, or suffering from a tumor in her brain or ovaries. High FSH levels may mean that her ovaries have failed, or that she no longer has any viable eggs in her ovaries. Or it could mean that she is experiencing menopause.

Other than stress or malnourishment, these effects will likely prevent most typical fertility interventions, and will require more intensive treatments. If a woman's ovaries have failed, or if the woman is unable to produce more eggs, she might be a candidate for in-vitro fertilization using a surrogate's eggs (unless some eggs can be extracted from her ovaries). In most cases, though, FSH is not the cause for most people's fertility struggles. The best thing to do if you think your FSH levels are too high or too