Converts testosterone (T) into 5-alpha-dihydrotestosterone (DHT) 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 Optimally active at acidic pHs. Belongs to the steroid 5-alpha reductase family.