When measuring testosterone levels, it is critical to determine the levels of both free and total testosterone to understand the cause of any observed symptoms of deficiency (Khosla et al 2008).
Because of difficulties with equipment standardization and inter-laboratory variability, it is recommended that physicians consistently use the same local laboratories and gain familiarity with the accuracy, precision and definition of normal values for the assays offered in their communities (Morales et al 2010).
Many hormones and their structural and functional analogs are used as medication . The most commonly prescribed hormones are estrogens and progestogens (as methods of hormonal contraception and as HRT ),  thyroxine (as levothyroxine , for hypothyroidism ) and steroids (for autoimmune diseases and several respiratory disorders ). Insulin is used by many diabetics . Local preparations for use in otolaryngology often contain pharmacologic equivalents of adrenaline , while steroid and vitamin D creams are used extensively in dermatological practice.
DHT is a potent agonist of the AR, and is in fact the most potent known endogenous ligand of the receptor. It has an affinity (K d ) of to nM for the human AR, which is about 2- to 3-fold higher than that of testosterone (K d = to nM)  and 15–30 times higher than that of adrenal androgens .  In addition, the dissociation rate of DHT from the AR is 5-fold slower than that of testosterone.  The EC 50 of DHT for activation of the AR is nM, which is about 5-fold stronger than that of testosterone (EC 50 = nM).  In bioassays , DHT has been found to be - to 10-fold more potent than testosterone.