The majority of circulating testosterone is bound to plasma protein (around 98%), rather than existing in free form . Of this, a majority is bound to a specific sex (or gonadal) steroid-binding protein , and a minority is bound to albumin. Dihydrotestosterone is produced from testosterone in the tissues by a specific enzyme, 5-alpha-reductase, rather than circulating in bound or free form. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 23. Function of the Male Reproductive System. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.