Dihydrotestosterone is a tissue metabolite of testosterone. It is formed under the influence of the 5a-reductase enzyme, which exists as two isoforms:
Type 1 5a-R
Is mainly found in the sebaceous glands and the epidermis, but is also present in sweat glands, hair follicles, endothelial cells and Schwann cells in the myelin sheaths of nerves.
Type 2 5a-R
Is mainly located in the hair follicles (inner layer, infundibulum, sebaceous glands). The conversion of circulating testosterone into DHT is mainly performed by isoenzyme 5α-R type 2. Dihydrotestosterone has the ability to bind to sex hormone binding globulin (SHBG) more than three times higher than testosterone. In men, approximately 70% of DHT is formed from the conversion of testosterone, while in women, the substrate is androstenedione.
Compared to Testosterone DHT has approximately fivefold greater affinity for the Androgen Receptor and DHT binding AR in hair follicles is commonly accepted as the first step leading to the miniaturization of follicles seen in AGA. The primary location for the AR in hair follicles is well known to be the dermal papillae in both anagen and telogen hairs
Oral 5-a-reductase inhibitor therapy has recently been shown to have long-term adverse effect in some patients (‘postfinasteride syndrome’)
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