What is FSH (follicle stimulating hormone)?
FSH is a pituitary hormone that signals the testes to produce sperm and is measured in men undergoing testosterone replacement therapy to evaluate fertility and testicular function.
FSH (follicle stimulating hormone) is a hormone produced by the pituitary gland that directly stimulates sperm production in the testes. In men, it works alongside luteinizing hormone (LH) and testosterone as part of the hypothalamic-pituitary-testicular axis, the system controlling male reproductive and sexual function.
During evaluation for testosterone replacement therapy, clinicians order FSH levels to assess how well the testes respond to hormonal signals. Low FSH combined with low testosterone may indicate a problem in the pituitary or hypothalamus rather than the testes themselves. Conversely, elevated FSH with low testosterone can suggest primary testicular failure. This distinction matters because it affects whether hormone therapy options will restore natural function or require ongoing treatment.
FSH levels are typically interpreted alongside LH and total testosterone to build a complete picture of a patient's hormonal status. Men who are candidates for TRT often have FSH testing done before treatment begins and periodically during therapy to monitor the testes' ongoing response. Some clinics also check FSH if a patient reports fertility concerns, since sperm production depends critically on adequate FSH stimulation.