Actions mediated by 2 progesterone receptors, PR-A and PR-B.

PR-A and PR-B which function as ligand activated modulators of gene expression.

Estrogens induce progesterone receptors.

Reduces the number and effects of alpha estrogen receptors.

Down regulates alpha estrogen receptors.

Protect against estrogen induced cell growth and counteracts many of estrogen effects.

Causes maturity and differentiation of endometrium and breast tissue.

Promotes apoptosis.

Decreases thyroid binding globulin and increases available thyroid hormone.

Is the dominant hormone of the second half of the menstrual cycle.

Measured in nanograms per mL while estrogens are measured in picograms per mL.

There are 1000 ng in a picogram.

Progesterone levels is always many times higher than estrogen levels, but it is lowest in the first half of the cycle.

Relaxes the myometrium by repressing the expression of genes encoding for contraction associated proteins, which promote labor.

When given in combination with estrogens counteracts the effect of estrogen on mood.

Megestrol, a synthetic progestin, has slight corticosteroid activity that becomes significant at high doses.

Essential to achieve and maintain a healthy pregnancy.

Progesterone is secreted by the corpus luteum during the second half of the menstrual cycle and by the corpus luteum and placenta during early pregnancy.

Progesterone prepares the endometrium for implantation of the embryo.

If implantation occurs, the corpus luteum continues to produce progesterone.

Between eight and 12 weeks of gestation the placenta maintains the pregnancy by secreting progesterone.

Progesterone therapy given in the first trimester in patients with a history of unexplained recurrent miscarriages does not result in significant higher rate of live births (Coomarasamy A et al).

Semi synthetic progestin megestrol has therapeutic effects for post menopausal women with metastatic breast cancer.

Proposed mechanisms of action in breast cancer include interaction with steroid receptors, reduced cellular estrogen uptake, and growth factor interactions, as well as suppression of the adrenal steroid production and ovarian secretion of androgens.

Megestrol for treatment of ER positive metastatic breast cancer is useful for a second or third line hormonal treatment option for patients who have relapsed on SERM and AI agents.

Progestogens promote the growth of uterine fibroids.


Side affects of megestrol are related to its anti-estrogen and anti-androgen effects and include: weight gain, edema, breakthrough menstrual bleeding and thromboembolic events.

Progestational agents used for hormonal contraception, or hormonal replacement therapy are associated with a higher chance of developing meningiomas.

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