• Estradiol is the usual active ingredient in the various brands of patches and gels, as well as the vaginal ring Estring. It is also a component of the conjugated estrogens - CES, Cenestin and Premarin. For information take the Which OHT link above
• Estrone too is an ingredient of conjugated estrogens, as well as a single ingredient in pill form. It is the type of estrogen which postmenopausal women continue to make the most.
• Estriol is the weakest of the three, also to be found in the conjugated estrogens as well as alone in pill form or a cream - especially in Europe. In North America it is not much used except by alternative practioners who often tout it as safer, and sometimes even as protective. See the rest of this page for how this is misleading.

Frank Z.  Stanczyk, PhD Professor of Research, Obstetrics & Gynecology, and Preventive Medicine Director, Reproductive Endocrine Research Laboratory University of Southern California Keck School of Medicine Los Angeles, California

The focus of this paper will be on the types of available estrogens and how they are classified and defined.  In particular, common misconceptions and myths about "natural" versus "synthetic" estrogens will be explored.  Much confusion has arisen, largely because the term "natural" has not been consistently applied.  The traditional classification scheme, presented here, defines the term narrowly.  However, the word "natural" carries with it many connotations, among them the belief that "natural is better."  This discussion will address the various ways that the term "natural" has been used to describe estrogens, sometimes irrespective of the origin or the number of chemical steps required to manufacture a final product from a natural source.

In the classification scheme contained in Table 1, natural steroidal estrogens are found in living organisms (plant or animal).  As pointed out earlier, this definition of natural does not denote or imply the source of the estrogen that is used for ERT.  Most of the steroidal estrogens that are used therapeutically are prepared from a steroid such as diosgenin in a series of chemical reactions.  These estrogens are sometimes marketed as "natural estrogens" because the precursor product (e.g., diosgenin) is derived from soybeans or yams.  However, the term "natural estrogen" as applied in this case neither follows the traditional classification scheme or reflects the fact that substantial chemical modifications are needed to create the final estrogenic product.  Table 2 shows a description of estrogens commonly used for ERT, according to the traditional classification scheme and source of the product.

More recent literature reports that when estriol is given in doses equivalent to estradiol, and administered more frequently to compensate for its rapid excretion, estriol induces endometrial hyperplasia. The impression that estriol exerted a protective effect was due to the fact that it has a very low biologic activity. It is rapidly cleared and binds to estrogen receptors with 1/3 to 1/2 of the affinity of estradiol. It does not appear to competitively interfere with estradiol binding, nor does it limit the histologic changes induced by estradiol. Biopsies of the endometrium in women who were treated with estriol only, estradiol only, and the two in combination showed similar histologic changes. Although breast cancer rates in premenopausal women correlate inversely with estriol excretion, in postmenopausal women high levels of estriol excretion, which correlate with serum levels, are associated with higher rates of breast cancer. No data indicate that exogenous supplementation with estriol lowers the incidence of postmenopausal breast cancer. 

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FINDINGS: 

After multivariate adjustment, oral use of oestriol 1-2 mg daily increased the relative risk of endometrial cancer and endometrial atypical hyperplasia: the odds ratios for at least 5 years of use compared with never use were 3.0 (95% CI 2.0-4.4) and 8.3 (4.0-17.4), respectively. The association was stronger for well-differentiated cancers and those with limited invasion. The excess relative risk was lost rapidly after cessation of treatment. Only weak associations were observed between vaginal application of low-potency oestrogen formulations and relative risk of endometrial neoplasia. 

Oral, but not vaginal, treatment with low-potency oestrogen formulations increases the relative risk of endometrial neoplasia. Thus close surveillance of patients is needed, and addition of a progestagen should be considered.

This study is also the basis of a "prescriber update" file on the NZ Medsafe site

Estriol is a weak estrogen which cannot be converted to estradiol. It is almost completely conjugated in the intestine to glucoronides and sulphates after oral intake; only 1-2% estriol reaches the circulation. Enterohepatic circulation can contribute significantly to the estriol levels, with food ingestion resulting in a secondary rise of estriol levels. It may therefore be advisable that estriol is taken in the evening in order to avoid endometrial proliferation during unopposed treatment. It is noteworthy that unexpectedly high systemic concentrations can be attained with vaginal applications (creams, ovules).

http://www.tnp.com/substance.asp?ID=1012
Estriol (short extract)

The estrogen tablets prescribed for menopausal symptoms usually contain estradiol, estrone, or a combination of the two. Some alternative medicine physicians have popularized the use of estriol as an alternative, and there is no doubt that estriol is also effective for symptoms of menopause. However, despite claims that it is safer than other forms of estrogen, the balance of evidence suggests that, in fact, estriol presents precisely the same risks (see Safety Issues below). 

Estriol: Safety and Efficacy by Kathleen A. Head, N.D
Abstract

While conventional hormone replacement therapy provides certain benefits, it is not without significant risks. Estriol has been found to provide some of the protection without the risks associated with stronger estrogens. Depending upon the situation, estriol may exert either agonistic or antagonistic effects on estrogen. Estriol appears to be effective at controlling symptoms of menopause, including hot flashes, insomnia, vaginal dryness, and frequent urinary tract infections. Results of research on its bone-density-maintaining effects have been contradictory, with the most promising results coming from Japanese studies. Estriol's effect on cardiac risk factors has also been somewhat equivocal; however, unlike conventional estrogen prescriptions, it does not seem to contribute to hypertension. Although estriol appears to be much safer than estrone or estradiol, its continuous use in high doses may have a stimulatory effect on both breast and endometrial tissue. (Alt Med Rev 1998;3(2):101-113)