Testosterone

Testosterone is the main androgen; androgens are the group of hormones it belongs to. It drives male sexual differentiation, meaning the body develops as male; male secondary sex characteristics, the physical traits associated with being male; spermatogenesis, the production of sperm; libido, or sex drive; and male fertility.

It is made in the testis, inside cells called Leydig cells, which build it from cholesterol. Luteinizing hormone acts on these cells and increases production. And here is the interesting part: testosterone puts the brakes on itself. When its blood level rises, it suppresses GnRH, the signal sent by the hypothalamus, and so it limits its own secretion. It works just like a home thermostat, which turns the heat down once the temperature has gone up.

It also acts on muscle and blood. It increases skeletal muscle mass, the muscles we move voluntarily. And it boosts erythropoiesis, the production of red blood cells. That explains why men have more hemoglobin, the protein that carries oxygen in the blood, than women.