The important role of the thyroid gland in human physiology and the incidence of thyroid dysfunction have led to increasing efforts to establish analytical criteria for thyroid function. Thus, there are more methods available for assessing thyroid function than for any other endocrine gland. Radioimmunoassay (RIA) employs the concept of competitive-protein binding, but the binding agent is a specific antibody raised against thyroxine rather than the thyroid-binding globulin (TBG) used in the Murphy-Pattee method. Thyroxine is dissociated from its binding proteins, and competes with added radioactively tagged thyroxine for limited antibody sites. The use of ion-exchange resins has been used to assay serum thyroid hormone levels. Using this technique, referred to as T4-by-column, thyroxine is dissociated from its carrier proteins with sodium hydroxide and applied to a strongly basic anion-exchange resin. The first enzyme-immunoassay developed for thyroxine analysis was the homogeneous Enzyme-Multiplied Immunoassay Technique (EMIT).