Sex steroid hormones

Testosterone is the principal androgen and is synthesized by the testes in the male. Oestradiol, which is secreted by the ovaries, varies widely in concentration in plasma throughout the female menstrual cycle. Steroids with oestradiol-like action are called oestrogens. Progesterone is also a product of the ovary and is secreted when a corpus luteum forms after ovulation. Normal female plasma also contains a low concentration of testosterone, about half of which comes from the ovary and half from peripheral conversion of androstenedione and dehydroepiandrosterone (DHA) sulphate, which are secreted by the adrenal cortex. Some oestradiol is present in low concentration in normal male plasma.

Testosterone and oestradiol circulate in plasma mostly bound to plasma proteins, particularly sex hormone-binding globulin (SHBG). The plasma concentration of SHBG in females is twice that in males. In both sexes the effect of an increase in SHBG is to increase oestradiol-like effects, whereas a decrease in SHBG increases androgen effects.

In females, testosterone and SHBG concentrations are sometimes reported by the laboratory as a ratio (the free androgen index), which gives a clearer indication of androgen status than does serum testosterone alone. In males, calculated free testosterone (using equations) is a more reliable indicator than measured total testosterone; free androgen index is not useful.

Hypothalamic–pituitary–gonadal axis

The episodic secretion of the hypothalamic hormone, gonadotrophin-releasing hormone (GnRH), stimulates synthesis and release of the gonadotrophins, LH (luteinizing hormone) and FSH (follicle-stimulating hormone), from the anterior pituitary. Despite the names, both gonadotrophins act cooperatively on the ovaries in the woman and the testes in the man to stimulate sex hormone secretion and reproductive processes.

Male gonadal function

The testes secrete testosterone and manufacture spermatozoa. Before puberty, gonadotrophin and testosterone concentrations in plasma are very low. The development of the Leydig cells and their secretion of testosterone is influenced by LH, whereas Sertoli cell function is influenced by FSH (Fig 50.1). Testosterone is responsible for the development of the male secondary sex characteristics such as hair growth, deep voice and characteristic musculature.

Female gonadal function

Oestradiol is responsible for:

Concentrations are low before puberty, but then rise rapidly and fluctuate cyclically during reproductive life. After the menopause, plasma oestradiol concentrations fall despite high circulating concentrations of the gonadotrophins.

The normal hormonal control of the menstrual cycle is shown in Figure 50.2. At the beginning of the cycle, FSH is released and initiates follicular growth. At mid-cycle a surge of LH triggers ovulation. The ruptured follicle differentiates into the corpus luteum that secretes progesterone and oestradiol whose target is the endometrium, which they prepare for implantation.