Unlike the case of estradiol, in which there is a rapid rise in its levels and which remain elevated in a plateau-like curve for many hours, levels of EE fall rapidly after peaking. This is thought to be because estrone and estrone sulfate can be reversibly converted back into estradiol and serve as a hormonally inert reservoir for estradiol, whereas the EE sulfate reservoir for EE is much smaller in comparison. In any case, due to the formation of EE sulfate, enterohepatic recirculation is involved in the pharmacokinetics of EE similarly to estradiol, although to a lesser extent. The contribution of enterohepatic recirculation to total circulating EE levels appears to be 12 to 20% or less, and is not observed consistently. A secondary peak in EE levels 10 to 14 hours after administration can often be observed with oral EE.