A New Beginning for Estrogen Physiology Editorial

Molecule for molecule, estradiol is the most potent of the steroid hormones. Its potency facilitated the design of bioassays for the early isolation and purification of the hormone. Furthermore, physiologically effective (and mean circulating) levels of hor-mones in general are in inverse proportion to the affinities of the hormones for their various receptors. The high affinity of estradiol for the estradiol receptor (Kd of- 0.1 nM) was an advantage in characterizing the receptor. As a consequence of these features, estradiol was one of the first steroid hormones to be purified and among the first for elucidation of the mecha-nism of action. The disadvantage of this high potency is that the hormone can act at concentrations that are not detectable by the best available assay procedures. This problem is not so serious for analysis of estrogen physi-ology in the mature female because, at ovulation, estrogen level