5aR1 (also known as SRD5A1) is an enzyme responsible for converting T into DHT mostly in tissue outside of the prostate such as the adrenal glands, brain, skin, digestive tract, liver, kidney, and thyroid. For this reason, 5aR1 is thought to be involved in circulating (serum) DHT levels. High DHT drives hair growth and eventually loss, enlargement of the prostate gland, bladder, and urinary problems and even certain cancers. As a result, the 5aR enzymes are the target of the anti-androgen drug, Dutasteride, prescribed to reduce hair loss and enlarged prostate gland.
5aR enzyme activity is reportedly naturally lower in Asian populations than African and Caucasians.
The C-allele result in the 5aR1 variant (rs1691053) is associated with increased serum DHT levels.