Reproductive Endocrinology

This lecture was mainly a refresher to get us stuck into the topic, so hopefully this post will be easy to write...! (Also- 500th post! 0_o)

Hormone Action

See previous post: Biochemical Messengers

Steroids

Steroids are lipid-soluble hormones that are derived from cholesterol. Some examples of steroid hormones include progesterone, aldosterone, cortisol, testosterone and oestradiol. As they do not dissolve well in water, roughly 95% of steroid hormone in the blood is bound to a carrier protein. The remaining 2-5% is free hormone that can diffuse into cells. Steroids usually bind to an intracellular receptor, which then diffuses into the nucleus and acts as a transcription factor. (For more details, see here.)

Progestagens

Progestagens are a type of steroid that includes progesterone. Progesterone is the most potent progestagen (i.e. has the highest affinity for its receptor). Other progestagens include 17α-hydroxyprogesterone (17α-OHP), which is around 40-70% as potent as progesterone, and 20α-hydroxyprogesterone (20α-OHP), which is only around 5% as potent as progesterone. Progestagens are particularly important in gestation, thickening of the endometrium in the second half of the menstrual cycle, and negative feedback on pituitary secretion of FSH and LH.

Androgens

Androgens include 5α-dihydrotestosterone (DHT), which is the most potent, testosterone, which is the most common but is only 50% as potent as DHT, androstenedione, which is 8% as potent as DHT, and dehydroepiandrosterone (DHEA), which is only around 4% as potent as DHT. Testosterone can be converted into DHT by 5α-reductase (see here for more information). Androgens are mainly important in the development of secondary sex characteristics, such as male genitalia, body hair, and so on. They are also anabolic and have negative feedback on pituitary secretion of FSH and LH.

Oestrogens

Oestrogens include oestradiol 17β, which is the most common and the most potent, oestriol, a common hormone in pregnancy which is only around 10% as potent as oestradiol, and oestrone, which is only around 1% as potent as oestradiol. Oestrogens are all formed from androgens via the action of an enzyme called aromatase. The functions of oestrogens include development of the endometrium during the first half of the menstrual cycle, increasing the water content of cervical mucus, and providing negative feedback to the pituitary.

Corticosteroids

Corticosteroids aren't normally associated with reproduction, but they may have some roles in the reproductive system. Corticosteroid levels increase during gestation and are important for fetal organ maturation. It has also been suggested that corticosteroids may be involved in the onset of parturition (labour), maternal behaviour, and suppression of the reproductive system, but further research is needed to confirm these.