Article Non-Monotonic Dose Responses in Studies of Endocrine Disrupt...
Generally, the U-shape dose-response curve is proposed for endocrine disruptors because they induce different mechanisms depending on the dose level. Different mechanisms are exposed to a complex dose-response curve.
Hormesis is a theory proposed and promoted by Ed Calabrese et alli who spent considerable effort looking for the "smoking gun" of a U shaped dose response curve. It is a compelling but fuzzy theory. See Toxicol_Sci_-2001-Calabrese-330-8.pdf (pehsu.net).