Steroid hormones are a subgroup of lipids (‘fat’ chemicals), which share a chemical structure characterised by four fused rings (Figure2.5). Each ring has five carbon atoms (C). Cholesterol (chole – bile, and steros- solid) was the first of this group to be purified and gave us the group name ‘steroids’. There are complex chemical pathways from cholesterol through to the sex steroids – progesterone, testosterone and the oestrogens. Testosterone is one of the male hormones, called androgens, and oestradiol is one of the female hormones called oestrogens. Men and women have both androgens and oestrogens although in women oestrogens predominate whereas in men there are more androgens. There is only one progesterone although synthetic ‘progestogens’ used in treatment regimens are similar in structure.