Testosterone

In men, testosterone is produced by the testes and is essential for sperm production. Low testosterone (hypogonadism) can reduce libido and sperm output. Importantly, exogenous testosterone (injections, gels, patches for bodybuilding or TRT) suppresses the hypothalamic-pituitary-gonadal axis and causes dramatic reduction in sperm count — often to zero.

In women, modestly elevated testosterone is a hallmark of PCOS, contributing to irregular ovulation, acne, and excess hair growth. Very high levels may indicate an ovarian or adrenal tumor.

Men wishing to preserve fertility should avoid exogenous testosterone; alternatives include clomiphene or hCG to stimulate the testes naturally.