What Are Hormones, and How Do They Work?

Hormones are long-range chemical messengers of the body, manufactured and controlled by the endocrine system. Hence the title of endocrinologist for hormone doctors.

The hypothalamus produces gonadotropin-releasing hormone (GnRH). This signals the anterior pituitary gland to synthesize and release luteinizing hormone (LH). To a lesser degree, GnRH also triggers the synthesis and release of follicle stimulating hormone (FSH). Subsequently, LH and FSH signal the gonads (ovaries in females, testes in males) to synthesize and release hormones that cause differentiation of the body tissue into female or male form: estrogen, progesterone, and testosterone. A small quantity of testosterone is also produced by the adrenal gland. Proportionally, females have more estrogen and progesterone than males; males have more testosterone.

Estrogens include natural and synthetic estradiols, estrones and estriols. They excite estrogenic receptors, causing the body to differentiate into female form and function. Natural and synthetic estrogens are hereafter referred to simply as estrogens.

Progestins/progestagens/gestagens (synonyms) are synthetic progesterone analogues. Progesterone and progestins excite progesteronic receptors, which in cooperation with estrogenic activity, cause the body to further differentiate into female form and function.

Various testosterones are collectively known as androgens. They excite androgenic receptors, causing the body to differentiate into male form and function. Natural and synthetic testosterones are hereafter referred to simply as androgens.

Anti-hormones can be useful in transsexual hormone therapy because they block hormone action or production. The basic mechanisms are:

• Androgen receptor antagonist: blocks the action of androgens at certain receptor sites.
• Androgen conversion inhibitor: blocks the conversion of one type of androgen to another.
• GnRH agonist: briefly overstimulates then effectively suppresses pituitary LH and FSH production.

Aggressive exogenous hormone therapy indirectly reduces endogenous (natural) gonadal hormone production by fooling the pituitary into thinking that there are plenty of hormones already in the body; consequently, the pituitary reduces the LH and FSH signals that stimulate the gonads.

Postnatally administered hormones do not cause development of genitals opposite those of birth. However, postnatal contrasexual hormone therapy does cause development of secondary sex characteristics as subsequently described in this document.

Next: What are normal endogenous androgen and estrogen levels?