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Steroid 5α-reductase reduces the conjugated double bond of particular steroids using NADPH. Human type 2 can convert testosterone to dihydrotestosterone and generate an inhibitory enzyme cofactor adduct from finasteride.

An androgen is a hormone that promotes development and maintenance of male sex characteristics. Endogenous examples include testosterone and dihydrotestosterone. The term identifies a class of biological signals, while each member has its own chemical structure and metabolic transformations.^[ref][ref]

Classifying a hormone by an important function does not specify every location where it is made: testosterone is produced prominently by the testis and also in smaller amounts by the ovary.^[ref]

Two conversions illustrate why the individual compound still matters. Reduction of testosterone produces the androgen dihydrotestosterone; aromatase instead can convert testosterone into the estrogen estradiol.^[ref] Following total steroid material would not distinguish those outcomes. A useful assay names the precursor, product and relevant response, rather than treating every related steroid as the same hormonal signal.

Dihydrotestosterone is the 5α-reduced steroid formed from testosterone. Reduction saturates the C4=C5 bond while retaining the C3 ketone and the other steroid functional groups.

Hydride is delivered to steroid C5 and a proton to C4; the C3 ketone is retained. Use the conjugated fragment $\mathrm{O}=C3-C4=C5$. Carbonyl hydrogen bonds from Tyr91–OH and protonated Glu57 lower the barrier to conjugate reduction. Draw a curved arrow from the nicotinamide C4–H bond of NADPH to steroid C5, shift the C4=C5 pi pair toward C3–C4, and shift C3=O pi electrons onto oxygen. The nicotinamide ring rearomatizes to NADP+; the steroid becomes a bound enolate, or its protonated enol equivalent. electron shifts across the nicotinamide ring must accompany C–H bond cleavage rather than leaving an unexplained neutral carbon radical.

Next, reform C3=O and protonate steroid C4 from an appropriately positioned acid, directly or through an enol/proton-relay route. This regenerates the catalytic acid after solvent reprotonation. The enzyme fixes the hydride-accepting face so that the new C5–H has the 5α configuration, giving dihydrotestosterone; delivery to the opposite face would give a different ring-junction configuration. This is not simple 1,2 reduction of the C3 carbonyl to an alcohol. The net local bookkeeping is one hydride plus one proton across C4=C5, with two electrons supplied by NADPH. The original reactive-fragment diagram shows the enolate-forming arrows and the separate protonation step.

Testosterone is a steroid with a conjugated C3-ketone/C4=C5 fragment. 5α-reductase adds hydride at C5 and a proton at C4 to make dihydrotestosterone.