E1S itself is biologically inactive, with less than 1% of the relative binding affinity of estradiol for the ERα and ERβ.

[5] Simultaneously, estrogen sulfotransferases, including SULT1A1 and SULT1E1, convert estrone to E1S, resulting in an equilibrium between the two steroids in various tissues.

[1][6][7] In accordance, E1S has been found to transactivate the estrogen receptor at physiologically relevant concentrations.

[8][9] Unlike unconjugated estradiol and estrone, which are lipophilic compounds, E1S is an anion and is hydrophilic.

[24][25] Studies in animals and humans have had mixed findings on uptake of exogenously administered E1S in normal and tumorous mammary gland tissue.

[26][27][28][24][25] This is in contrast to substantial uptake of exogenously administered estradiol and estrone by the mammary glands.

[26] Another animal study found that E1S wasn't taken up by the uterus but was taken up by the liver, where it was hydrolyzed into estrone.