Testosterone

UNDER REVIEW (September 2016)

Mechanism  of Action:

The principal androgen, or male sex hormone. One of the group of compounds known as anabolic steroids, testosterone is secreted by the testes but is also synthesized in small quantities in the ovaries, cortices of the adrenal glands, and placenta, usually from cholesterol. Testosterone is necessary in the fetus for the development of male external genitalia; increased levels of testosterone at puberty are responsible for further growth of male genitalia and for the development and maintenance of male secondary sex characteristics such as facial hair and voice changes. Testosterone also stimulates protein synthesis and accounts for the greater muscular development of the male. For many years, synthetic steroids similar to testosterone have been used by athletes with the goal of improving performance, although recent medical research has shown that these drugs may have a wide range of harmful side effects.

Progesterone

UNDER REVIEW (September 2016)

Action of Mechanism:

Progesterone, a female sex hormone, is secreted by the corpus luteum in the ovary after the rupture of the follicle during ovulation (mid-cycle). It fertilization occurs, the placenta takes over the secretion of progesterone after around 8 weeks, whilst the corpus luteum degenerates. If fertilization does not occur, the corpus luteum undergoes luteolysis after 2 weeks, causing progesterone levels to fall. Progesterone is important for vascularisation and final growth of the endometrium, which increases in size and number of secretory glands. These preparations make the endometrium suitable for implantation by the blastocyst (fertilized egg). If implantation does occur, progesterone is essential for maintenance of pregnancy, preventing spontaneous abortion. Otherwise, progesterone levels fall, causing menstruation (shedding of endometrium). This is because progesterone withdrawal is linked with the entry of leukocytes into endometrium which produces inflammatory mediators eg prostaglandins and cytokines. Endometrial blood vessel constrict, causing reduced oxygen and blood supply (ischaemia) which results in tissue death and endometrium shedding. Progesterone exerts its effects by acting on intracellular receptors and changing gene expression. It is also made from cholesterol in the adrenal cortex, where it is a precursor of testosterone synthesis. Progesterone has a role in milk production by mammary glands. Synthetic progesterones include: progesterone derivatives – dydrogesterone, medroxyprogesterone, testosterone derivatives – norethisterone, norgestrel, desogestrel, norgestimate, gestodene.

Oestrogen

Mechanism of Action:

A group of steroid hormones (including oestriol, oestradiol and oestrone) synthesized by the reproductive organs (ovary and placenta) and adrenal glands in females and, in smaller quantities, in males (testis). The main oestrogen produced by the ovary is 17β;-estradiol, which is converted to oestrone in the blood. Oestriol is the main oestrogen produced by the placenta in pregnancy. Oestrogen exerts its effects by acting on intracellular oestrogen receptors. Oestrogens promote the growth of sexual glands and female secondary sexual characteristics (e.g. pubic hair and breasts). Oestradiol plays an important role in the menstrual cycle: in the early stages, it causes proliferation of the endometrium in the uterus and inhibits FSH. At mid-cycle, it stimulates LH release and causes the cervical mucus to be thinner and less hostile to sperm. Other effects of oestrogens are exerted throughout the body and include maintaining vessels and skin, promoting bone formation, increasing HDL levels and decreasing LDL. Oestrogen-like drugs include tibolone (oestrogenic, progestogenic and weak androgenic activity) and the selective oestrogen receptor modulators (raloxifene and tamoxifen) which have pro-oestrogenic effects in some organs but antioestrogenic effects in others.

Lecture and CAL materials: