Luteinizing hormone and follicle-stimulating hormone are pituitary glycoproteins that drive gonadal steroidogenesis.
Mechanism: Both are released in response to hypothalamic GnRH. In men, LH stimulates Leydig cell testosterone production; FSH supports Sertoli cell sperm maturation. In women, they drive the menstrual cycle (LH triggers ovulation; FSH grows follicles).
Lab range: Men: LH 1.7–8.6, FSH 1.5–12.4 mIU/mL. Women: varies dramatically by cycle phase.
Caveats: Low LH/FSH with low testosterone = secondary hypogonadism (pituitary or hypothalamic origin). High LH/FSH with low T = primary hypogonadism (testicular). TRT crashes both unless paired with HCG (an LH-mimetic).