Synthesis

Oestrogens are cholesterol derivatives; the main endogenous form, oestradiol-17beta, is synthesised via androgen intermediates. Ovarian synthesis, the major source outside of pregnancy, has two major routes of production:

• the theca interna cells are able to carry out the entire synthesis from cholesterol to androstenedione to oestradiol
• additionally, the granulosa cells are able to synthesise oestradiol from androstenedione. Androstenedione is received from the theca interna cells or from the circulation. Androstenedione within the circulation is derived from the adrenal cortex.

The stromal tissues of the ovary may also be able to biosynthesise oestrogens; however, their contribution is likely to be insignificant.

Synthesis increases from the start of the ovarian cycle until approximately day 14 with ovulation. The peripheral plasma concentration of oestradiol-17beta at this time is roughly 4-6 times the baseline at the start of the cycle. During pregnancy, oestradiol levels may rise to 80-100 times the ovarian cycle baseline.