Estrone-d2 is intended for use as an internal standard for the quantification of estrone by GC- or LC-MS. Estrone is one of the three naturally occurring estrogens, the others being estradiol and estriol .1 Estrone is synthesized from androstenedione by the aromatase enzyme system in the ovaries and placenta, and is also synthesized from estradiol by 17-hydroxy steroid dehydrogenase in the liver.1,2 Serum concentrations of estrone in premenopausal women fluctuate according to the menstrual cycle and becomes the most predominant estrogen in postmenopausal women.1 The binding affinities of estrone to the estrogen receptors α and β are approximately 60% and 37% relative to estradiol.1

References
1. Gruber, C.J., Tschugguel, W., Schneeberger, C., et al. Production and actions of estrogens. New England Journal of Medicine 346(5), 340-352 (2002).