Natural sex hormones are made by the gonads (ovaries or testicles),[3] by adrenal glands, or by conversion from other sex steroids in other tissue such as liver or fat.

[4] In many contexts, the two main classes of sex hormones are androgens and estrogens, of which the most important human derivatives are testosterone and estradiol, respectively.

Other contexts will include progestogens as a third class of sex steroids, distinct from androgens and estrogens.

[5] Progesterone is the most important and only naturally occurring human progestogen.

Some systemic effects of a particular steroid in the entire organism are only desirable within certain limits.