The Role of FSH and LH in Male Fertility
How These Hormones Work Together
FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone) are the two master control hormones that tell your testicles to make sperm and testosterone—without them working properly, male fertility cannot happen. 1, 2
What LH Does
- LH acts like a "start button" for testosterone production by stimulating specialized cells in the testicles called Leydig cells to manufacture testosterone 2, 3
- Testosterone levels inside the testicles need to be 50-100 times higher than in the bloodstream to support normal sperm production, and LH is responsible for creating these high local concentrations 1
- Without adequate LH signaling, testosterone production drops and sperm production fails 4
What FSH Does
- FSH is the primary hormone that drives sperm cell development and maturation by acting on Sertoli cells, which are the "nurse cells" that support developing sperm 1, 2
- FSH determines how many Sertoli cells develop during puberty, which sets the maximum sperm production capacity for life 2
- FSH stimulates all phases of sperm development, from the earliest immature cells through to mature sperm 2, 4
- When sperm production is failing, FSH levels rise as the brain tries to compensate by sending stronger signals to the testicles 1, 5
How the Brain Controls These Hormones
- The hypothalamus (a brain region) releases GnRH (gonadotropin-releasing hormone) in pulses, which travels to the pituitary gland and triggers the release of both FSH and LH 4
- This system works like a thermostat with feedback loops: when testosterone and sperm production are adequate, the brain reduces FSH and LH output; when production is low, the brain increases these hormones 4
- Taking external testosterone completely shuts down this system through negative feedback, causing FSH and LH levels to drop to near zero, which stops sperm production entirely 1, 6
What Happens When These Hormones Are Abnormal
High FSH Levels (Above 7.6 IU/L)
- Elevated FSH typically signals that the testicles are struggling to produce sperm, prompting the brain to send stronger signals 1, 7
- Men with FSH above 7.6 IU/L often have reduced sperm counts or complete absence of sperm (azoospermia), though up to 50% may still have some sperm that can be surgically retrieved 1, 7
- The higher the FSH, generally the worse the sperm production, though FSH levels alone cannot definitively predict whether any sperm are present 1, 7
Low FSH and LH Levels
- When both FSH and LH are low, this indicates the brain is not sending proper signals to the testicles (called hypogonadotropic hypogonadism), which can be treated with hormone injections to restore fertility 1
- This pattern is completely different from high FSH, which indicates the testicles themselves are damaged 1, 7
Why Both Hormones Are Essential
- Neither FSH nor testosterone alone can maintain fully normal sperm production—both are required working together 2, 3
- FSH primarily drives the early stages of sperm cell division and proliferation, while testosterone is crucial for the later maturation stages 2, 4
- Studies in men with rare genetic mutations show that FSH alone can maintain some sperm production, but the combination of FSH and testosterone produces the best quality and quantity of sperm 2
Common Clinical Pitfalls
- Never take testosterone supplements if you want to preserve fertility—external testosterone causes FSH and LH to plummet, shutting down sperm production for months to years 1, 7, 6
- A single abnormal FSH or LH test can be misleading due to natural fluctuations, illness, obesity, or stress—repeat testing after addressing reversible factors is essential 1, 7
- FSH levels between 7.6-12 IU/L represent a gray zone where some men still produce adequate sperm while others have significant impairment—semen analysis is mandatory to determine actual fertility status 1, 7