Estriol

Estriol (E3) is a so-called "weak" estrogen that Ray Peat argued is not protective, but rather a slightly weaker variant of estradiol that produces the same toxic effects at a proportionally higher dose. 3 Peat emphasized that estriol is about 10 times weaker than estradiol, but that people simply use 10 times more to achieve an equivalent effect, making it functionally identical in its capacity for harm. 2 He stated plainly that "they're all interchangeable. It just takes a bigger dose of either estrone or estriol to have exactly the same effect as estradiol." 6

The notion that estriol is a benign or protective estrogen was a central target of Peat's critique. He traced the idea to a 1978 JAMA editorial promoting estriol for cancer resistance, which he dismissed as "aggressively stupid." 3 Peat cited evidence that estriol stimulates the growth of human breast cancer cells in culture and overcomes the antiestrogenic effects of tamoxifen, leading researchers to conclude that "the data do not support an antiestrogenic role for estriol in human breast cancer." 2 He further noted that the elevated conversion of estradiol to estriol, via 16-hydroxylation, is a hormonal feature found in women with breast cancer and in men who had a myocardial infarction, suggesting that high estriol is a consequence of disease, not a cause of protection. 2

Peat explained that during a healthy pregnancy, the placenta converts the highly toxic estradiol into the more water-soluble estriol, which is more quickly excreted in the urine, making a high estriol level an index of rapid estradiol detoxification rather than a beneficial hormone in itself. 3 A surge in estriol preceding labor was interpreted by Peat as likely reflecting the mother's increased production of adrenal androgens during stress, analogous to the hormonal dynamics of polycystic ovary syndrome. 2 He maintained that the decline of estriol in a failing pregnancy is a consequence of fetal sickness, not its cause. 2

Despite its reputation as a weak estrogen, Peat documented that estriol exerts potent systemic effects. Intravaginal administration of 5 mg suppressed LH within two hours and FSH within five hours, while oral dosing produced estrogenic changes in the vaginal epithelium. 3 Its anti-progestational activity is approximately the same as estradiol's, and when placed locally in the uterus of a rabbit, only 1.25 mcg was sufficient to prevent implantation and destroy the blastocyst. 3 Subcutaneous administration induced abortions and stillbirths. 3 Peat also warned that topical application for cosmetic purposes merely causes the skin to take up water, plumping it while altering collagen, an effect he characterized as "pro-aging." 4 Georgi Dinkov has extended this critique by arguing that locally applied estriol inevitably distributes systemically, citing the phenomenon of microchimerism as evidence that molecules far larger than estriol traverse the body, and noting that even weak estrogens applied to the skin can drive melanoma formation. 9