Sex hormones are the steroid hormones secreted mainly by the gonads (testes, ovaries) and the adrenal cortex, with testosterone, estrogen, and progesterone as the representative three. They are involved in a wide range of functions: the expression of secondary sexual characteristics, libido, reproductive capacity, bone density, muscle mass, and mood.

Principal sex hormones

Testosterone

Testosterone is the principal male sex hormone (an androgen), produced by the Leydig cells of the testis, with a small amount produced by the female ovary and adrenal. Its main functions: maintaining muscle mass and bone density, spermatogenesis, growth of pubic and axillary hair, deepening of the voice, regulation of libido, and an association with aggression and competitive behaviour (debated). Blood testosterone in men peaks in the 20s and declines gradually thereafter. Low testosterone (late-onset hypogonadism, “male menopause”) is associated with reduced libido, erectile dysfunction, fatigue, and low mood, and testosterone replacement therapy is used to improve these.

Estrogen

Estrogen is the principal female sex hormone (follicular hormone), produced by the ovarian follicles, with a small amount from the male testis and adipose tissue. Its main functions: breast development, endometrial proliferation, maintenance of bone mass, skin elasticity, vaginal lubrication, regulation of the menstrual cycle, and influence on libido. The sharp post-menopausal drop in estrogen causes hot flashes, osteoporosis, vaginal atrophy, and dyspareunia, managed with hormone replacement therapy (HRT).

Progesterone

Progesterone is secreted by the corpus luteum formed after ovulation, with a principal role in preparing the endometrium for implantation and maintaining pregnancy. It is also the hormone responsible for the rise in basal body temperature, and a principal component of oral contraceptives.

Sex hormones and libido

In both sexes, testosterone is confirmed as an important regulator of libido. In men, blood testosterone correlates with libido and frequency of sexual behaviour, and a similar relationship is reported in women (though some research finds the correlation weaker in women than in men). The effect of testosterone on libido is attributed to direct action on the regions of the brain regulating desire (the hypothalamus and others); the mechanism by which androgen-receptor-bearing neurons respond to testosterone and raise sexual motivation is under study.

Exogenous sex hormone administration

Hormone administration for medical or physical reasons is widely practised, including gender-affirming hormone therapy for transgender people (feminising and masculinising therapy). Testosterone replacement therapy is prescribed for men with low testosterone presenting erectile dysfunction, reduced libido, and reduced muscle strength, but requires management of side effects such as the relationship to prostate cancer risk and polycythaemia. Anabolic steroids including testosterone are listed as prohibited substances in sports competition, a context distinct from the use of testosterone for sexual purposes.

Related terms

• Libido
• Ovary
• Testicles
• Erectile dysfunction