Bioidendical hormone replacement therapy is the use of supplemental doses of naturally occurring sex steroid hormones. This is a modification of conventional hormone replacement therapy. Conventional hormone replacement therapy often involves the use of chemicals that have hormone activity but are not normally found in the human body (examples: synthetic progestins, equilin).

The term "bioidentical" is used because the administered hormones are chemically synthesized and identical to the naturally occurring hormones of the human body: estradiol, progesterone, estriol (another natural estrogen), and testosterone are the most common.

The sex steroid hormones can be administered orally, but when administered in this way most of the hormone is destroyed by the liver soon after entering the body. Particularly for progesterone, the resulting metabolic by-products can cause unwanted side-effects. In the case of estrogens, oral administration can alter the production of clotting factors by the liver, increasing the risk of dangerous strokes. For these reasons, topical administration of sex steroid hormones in increasingly popular. Some hormones have been made available as manufactured transdermal patches, particularly estradiol. Progesterone and estriol are mostly available in the form of topically applied creams.

Bioidendical hormone replacement therapy has received increasing attention since the termination of the Women's Health Initiative hormone replacement therapy clinical trials [1] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15354510). A recent book by Suzanne Somers, The Sexy Years: Discover the Hormone Connection, has also served to publicize bioidendical hormone replacement therapy.

Reasons why bioidendical hormone replacement therapy might have advantages over conventional hormone replacement therapy.

• Emphasis on topical administration; avoids problems such as blood clotting that are caused by the rapid metabolism of orally administered hormones [2] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=12927428).
• Naturally occurring progesterone may have a different risk/benefit spectrum than synthetic progestins [3] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=15358673).
• Emphasis on individualized doses rather than "one dose fits all" approach of conventional hormone replacement therapy [4] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12478948).
• Inclusion of estriol may be protective against hormone-induced cancer. Unlike estradiol, estriol binds preferentially to the second estrogen receptor (ERbeta). ERbeta may function as a tumor suppressor [5] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15369453).