FSH stimulates Sertoli cells in the testes, driving spermatogenesis (sperm production). Like LH, FSH is produced by the pituitary and suppressed by exogenous testosterone. Baseline FSH levels, along with LH, help classify hypogonadism.
Men concerned about fertility on TRT should monitor FSH, as its suppression correlates with reduced sperm production. HCG and enclomiphene can partially maintain FSH-driven spermatogenesis during TRT.