Epipregnanolone, also known as 3β-hydroxy-5β-pregnan-20-one, 3β,5β-tetrahydroprogesterone, or 3β,5β-THP, is an endogenous neurosteroid.

[1] It acts as a negative allosteric modulator of the GABAA receptor and reverses the effects of potentiators like allopregnanolone.

[2][3] Epipregnanolone is biosynthesized from progesterone by the actions of 5β-reductase and 3β-hydroxysteroid dehydrogenase, with 5β-dihydroprogesterone as the intermediate in this two-step transformation.

[2] Epipregnanolone is not a progestogen itself, but via metabolization into other steroids, behaves indirectly as one.

[4] The sulfate of epipreganolone, epipregnanolone sulfate, is a negative allosteric modulator of the NMDA[5] and GABAA receptors[6] and also acts as a TRPM3 channel activator.