The FTI is considered a more reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding. The FTI is obtained by multiplying the (Total T4) times (T3 Uptake) to obtain an index. It is elevated in hyperthyroidism and depressed in hypothyroidism. ![]() Free T4 is the most accurate assessment of thyroid hormone levels, because it avoids changes in plasma protein binding capabilities. Some (about 5%) is not bound (“free”) and available to serve a metabolic role. Most thyroid hormone is bound to plasma proteins. ![]() T3 is mostly bound to plasma proteins, and the remainder (FT3) is unbound. It has nothing to do with T3, despite the name.įree T3 is one of the determinations used to measure thyroid function. This is an indirect measure of thyroid hormone levels. The less thyroid hormone that is present, the more unsaturated TBG is present. This is a measure of the unbound throxine binding globulins in the blood (unsaturated with thyroid hormone). T3 (Triiodothyronine, T3-RIA) by radioimmunoassay T3 is metabolically a more active form of thyroid hormone than T4, but its’ effects are of shorter duration. T3 (Triiodothyronine, T3-RIA) by radioimmunoassay It is usually slightly elevated during pregnancy because of increased levels of TBG. ![]() 95% of the total is bound to plasma proteins, particularly thyroid binding globulin (TBG). This is a measure of all of the active form of thyroid hormone in the blood. It is elevated in hypothyroidism and depressed in hyperthyroidism. For the same reason, whenever there is too much thyroid hormone present in the serum, the brain responds by lowering the TSH. Thyroid function tests are a series of measurements of different aspects of thyroid hormone presence in the blood stream.īecause of the normal feed-back loop, TSH rises whenever the thyroid gland fails to produce sufficient thyroid hormone.
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