Dr. Shargani: The blood levels of DHT may play a minor role in Androgenetic Alopecia but it is the locally produced DHT which plays the major role. If you look at any physiology text book (such as Guyton's Medical Physiology) you can see that the majority of DHT production takes place at the site where it exerts its effects.

Also there is a high concentration of type 2 isoenzyme within the hair follicle itself and a higher concentration of the type 1 form within the scalp and sebaceous glands which feed into the hair follicle.

Together they can produce more DHT locally than what blood brings to the hair follicle. Also most researchers now believe that it is the locally produced DHT that is involved in Androgenetic Alopecia rather than the circulatory DHT. Finally there is new evidence about effectiveness of topical inhibition of DHT on hair growth by Dr. Christiano which provides support for this theory.