LH is produced by the pituitary gland and stimulates Leydig cells in the testes to produce testosterone. LH levels are critical for diagnosing the cause of low testosterone. High LH with low T indicates primary hypogonadism (testicular failure), while low LH with low T indicates secondary hypogonadism (pituitary or hypothalamic issue).
On TRT, LH drops to near-zero as exogenous testosterone suppresses pituitary gonadotropin secretion.