T Uptake (TU) and total T4 are used to estimate the amount of circulating free T4. The estimate, or the Free Thyroxine Index (FTI), is a normalized measurement that remains relatively constant in healthy individuals and compensates for abnormal levels of binding proteins, which can occur in many different physical conditions. Drugs or physical conditions that alter the patient’s TBG (thyroxine binding globulin) levels or drugs that compete with endogenous T4 and T3 for protein-binding sites alter T Uptake results. When serum contains high levels of T3 or T4, as in hyperthyroidism, fewer unoccupied binding sites are available. Conversely, in hypothyroidism, more binding sites are available.