3-Hydroxysteroid dehydrogenase
3-Hydroxysteroid dehydrogenase/5-4 isomerase (3-HSD) (EC 1.1.1.145) is an enzyme that catalyzes the biosynthesis of the steroid progesterone from pregnenolone, 17-hydroxyprogesterone from 17-hydroxypregnenolone, and androstenedione from dehydroepiandrosterone (DHEA) in the adrenal gland. It is the only enzyme in the adrenal pathway of corticosteroid synthesis that is not a member of the cytochrome P450 family. It is also present in other steroid-producing tissues, including the ovary, testis and placenta.  In humans, there are two 3-HSD isozymes encoded by the HSD3B1 and HSD3B2 genes.
3-HSD is also known as delta 5-4-isomerase, which catalyzes the oxidative conversion of 5-3-hydroxysteroids to the 4-3-keto configuration and is, therefore, essential for the biosynthesis of all classes of hormonal steroids, namely progesterone, glucocorticoids, mineralocorticoids, androgens, and estrogens.
The 3-HSD complex is responsible for the conversion of:
- Pregnenolone to progesterone
- 17-Hydroxypregnenolone to 17-hydroxyprogesterone
- DHEA to androstenedione
- Androstenediol to testosterone
- Androstadienol to androstadienone

# Reaction
3-HSD belongs to the family of oxidoreductases, to be specific, those acting on the CH-OH group with NAD+ or NADP+ as acceptor. This enzyme participates in C21-steroid hormone metabolism and androgen and estrogen metabolism.
3-HSD catalysis|catalyzes the chemical reaction:
Thus, the two substrates of this enzyme are 3-hydroxy-5-steroid and NAD+, whereas its three products are 3-oxo-5-steroid, NADH, and H+.

# Isozymes
Humans express two 3-HSD isozymes, HSD3B1 (type I) and HSD3B2 (type II). The type I isoenzyme is expressed in placenta and peripheral tissues, whereas the type II 3-HSD isoenzyme is expressed in the adrenal gland, ovary, and testis.

# Nomenclature
The systematic name of this enzyme class is 3-hydroxy-5-steroid:NAD+ 3-oxidoreductase. Other names in common use include:
- progesterone reductase
- 5-3-hydroxysteroid dehydrogenase
- 3-hydroxy-5-ene steroid dehydrogenase
- 3-hydroxy steroid dehydrogenase/isomerase
- 3-hydroxy-5-C27-steroid dehydrogenase/isomerase
- 3-hydroxy-5-C27-steroid oxidoreductase
- 3-hydroxy-5-ene-steroid oxidoreductase
- steroid-5-3-ol dehydrogenase
- 3-HSDH
- 5-ene-3-hydroxysteroid dehydrogenase
- 3-hydroxy-5-ene-steroid dehydrogenase

# Inhibitors
3-HSD is potently inhibited by azastene, cyanoketone, epostane, and trilostane. Medroxyprogesterone acetate and medrogestone are weak inhibitors of 3-HSD which may substantially inhibit it at high dosages.

# Biosynthetic pathway
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Human steroidogenesis, showing reactions of 3-HSD near-left in green box.
- Corticosteroid biosynthetic pathway in the rat, showing reaction catalyzed by 3-HSD (second arrow from the top).

# Clinical significance
A deficiency in the type II form through mutations in HSD3B2 is responsible for a rare form of congenital adrenal hyperplasia. No human condition has yet been linked to a deficiency in the type I enzyme. Its importance in placental progesterone production expression suggests that such a mutation would be embryonically lethal.
The fetal adrenal cortex lacks expression of the enzyme early on, thus mineralocorticoids (e.g. aldosterone) and glucocorticoids (e.g. cortisol) cannot be synthesized. This is significant because cortisol induces type II pneumocytes of the lungs to synthesize and secrete pulmonary surfactant; without pulmonary surfactant to reduce the alveolar surface tension, premature neonates may die of neonatal respiratory distress syndrome. If delivery is unavoidable (e.g. because of placental abruption, or pre-eclampsia/HELLP syndrome), then glucocorticoids (e.g. cortisol) can be administered.