Converts testosterone into 5-alpha-dihydrotestosterone and progesterone or corticosterone into their corresponding 5-alpha-3-oxosteroids. It plays a central role in sexual differentiation and androgen physiology. a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH 5alpha-pregnane-3,20-dione + NADP(+) = H(+) + NADPH + progesterone 17beta-hydroxy-5alpha-androstan-3-one + NADP(+) = H(+) + NADPH + testosterone androst-4-ene-3,17-dione + H(+) + NADPH = 5alpha-androstan-3,17-dione + NADP(+) Belongs to the steroid 5-alpha reductase family.