Progesterone is a sex hormone that works primarily to balance the potent effects of estrogens in the body. Specifically, progesterone counteracts the proliferative (tissue growth) effects of estrogen on hormonally sensitive tissue, such as breast and uterus, and selectively balances the effects of estrogen on other tissues including the brain, bones and skin. Unopposed estrogen (also known as estrogen dominance) occurs when there is insufficient progesterone to counteract estrogen.
Progesterone, which is very high during pregnancy, is responsible for keeping a fertilized egg viable, which is why low levels are often associated with infertility. While estrogen increases the immune response, progesterone depresses immunity, which is good because otherwise a woman would expel a fertilized egg as a foreign invader. In a premenopausal woman, large amounts of progesterone are released in the second half of her menstrual cycle and remain high if pregnancy occurs. It is often cited as the “feel-good” hormone responsible for the “glow” of pregnancy. If a woman doesn’t ovulate, her progesterone levels may be low. In post-menopausal women, progesterone levels are low relative to premenopausal years.
While estrogen prevents bone loss, progesterone promotes bone formation by stimulating osteoblasts (bone cells) and exerts neuroprotective effects on the brain. Natural progesterone tends to have a calming effect on mood and emotions. Studies indicate progesterone may be beneficial in treating brain injuries since it reduces enzymes that cause inflammation in the brain after injury or stroke. Progesterone shows a synergistic effect with vitamin D, which is actually a precursor to all sex hormones, on quelling inflammation in brain tissue. Evidence also suggests that vitamin C may increase progesterone levels. Progesterone selectively inhibits estrogen in perfectly orchestrated hormonal balance. Similarly, thyroid hormones and progesterone interact with each other to regulate metabolism.
Also, progesterone inhibits the enzyme aromatase, which converts testosterone to estrogen. As a result, progesterone may serve to conserve testosterone and increase libido in some women. In men, the reduction of this enzyme by progesterone may help reduce hyperplasia (tumors) in the prostate. Men typically have progesterone levels similar to levels seen in women in the first half (follicular phase) of their period.
It’s important to remember that progesterone refers to the naturally occurring hormone found in the human body and should not be confused with progestins, which are synthetic and do not have the same biological activity. The effects of progesterone are often totally different than progestins, which are typically found in oral contraceptives and conventional hormone replacement.
