What are Bioidentical Hormones?

BHRT Female Hormones

What are Bioidentical Hormones?

Women and men experience a decline in hormone production as they age. For women, the most notable are the drops in estrogen and progesterone levels. These changes become painfully apparent during menopause. While some women breeze through it with few or no symptoms, many experience annoying hot flashes and even depression.

Bioidentical hormone replacement refers to the practice of taking hormones which are identical to the ones produced by the human body. Bioidentical progesterone creams, for example, contain progesterone that is identical to the form produced by a woman's ovaries and adrenal glands. In contrast, the prescription drug Provera, known as medroxyprogesterone acetate, is a synthetic, non-identical form of progesterone.

Provera was combined with estrogens (derived from horse urine) to produce Prempro. After years of widespread use, it was found to be associated with an increased risk of heart attack and breast cancer in the Women's Health Initiative (WHI) study.

The findings of the WHI have been recently called into question. Some doctors believe the harm caused by hormone deficiencies is far worse than the potential risks of restoring them. Others have observed that many of the WHI participants started hormone replacement at an age when female hormone receptors were already lost. But while the use of conventional hormone replacement continues to be debated, many women have turned to a safer and more beneficial alternative.

During the 1930s, Professor Russell E. Marker of Pennsylvania State University transformed diosgenin from Mexican wild yam (Dioscorea villosa) into bioidentical progesterone.1 In the 1970s, the process of micronization (which consists in reducing the average diameter of solid particles so that they become only a few micrometers in diameter) led to a more effective bioidentical form, due to its steady and even absorption.2

While micronized progesterone has been available in creams, suppositories, and gels for years, it was approved in capsule form as Prometrium® in 1998. However, many believe that progesterone is better absorbed and has fewer side effects if taken topically. And because orally administered hormones are processed by the liver, Prometrium® may not be recommended for women with liver conditions.

Topical progesterone is better absorbed transmucosally, i.e., intravaginally. A trial that compared the effects of Prometrium® to an intravaginally administered progesterone gel found greater bioavailability and absorption, and less variability, for the transmucosal preparation.3 If progesterone is not administered transmucosally, it is recommended to apply it to fatty areas of the body and to rotate the areas of application.

Unlike capsules, one can easily modify the dosing of topical hormones. It is recommended that women have their blood tested prior to starting hormone replacement therapy. This helps to determine the safest and most effective dose. However, serum progesterone levels may not always reflect tissue levels of the hormone.4

For menstruating women, topical bioidentical progesterone may be taken during the last 16 days of the cycle to optimize levels. Progesterone levels may decline during perimenopause (the years prior to menopause). After menopause, women can apply progesterone once or twice daily for 7 to 21 days out of the month.

While a minimum of 7 days of progesterone treatment is suggested to protect the uterus from the effects of estrogen replacement therapy, many postmenopausal women use progesterone for up to three weeks per month to obtain other benefits, including its sleep-promoting effects.


  1. Yakushigaku Zasshi. 2001;36(1):70-5.
  2. Fertil Steril. 1999 Sep;72(3):389-97.
  3. Fertil Steril. 2000 Mar;73(3):516-21.
  4. Menopause. 2005 Mar;12(2):232-7.

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