Converts testosterone (T) into 5-alpha-dihydrotestosterone (DHT) and progesterone or corticosterone into their corresponding 5-alpha-3-oxosteroids (PubMed:1527072). It plays a central role in sexual differentiation and androgen physiology (By similarity). a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH Optimally active at acidic pHs. Expressed in high levels in the prostate and many other androgen-sensitive tissues. Belongs to the steroid 5-alpha reductase family.