AR = 5-alpha-reductase. The function of this enzyme is to convert testosterone into dht. Dht is basically 2 testosterone molecules glued together. Dht is what is binds to receptors in the hair follicle which starts a chain reaction which in the end strangles the follicles. Finasteride inhibits AR by binding competitively with testerone. Since both testosterone and finasteride bind the same place in the AR enzyme, they compete to bind AR and that makes finasteride an antagonist. So when you put enough finasteride in your body to outcompete testerone, AR can't covert any meaningful amount of testerone to DHT, which means we have a reduction in dht.
the reason you still have dht is because finasteride doesn't inhibit another type of AR enzyme also present in the body. Dutasteride inhibits both types which results in much less dht in the body.
