These assays have been variously referred to as T3-uptake, T4-uptake and thyroid-uptake tests, depending on the assay design. All are used in exactly the same manner and for the same purpose, not as stand-alone assays, but in combination with total T4 or total T3 assays. Matched T-uptake and total T4 results are used to calculate a free thyroxine index (FT4I or FT3I). The FT4I serves as an indirect estimate of free T4 levels, and were heavily relied upon historically, particularly before direct free T4 assays became available. The resin T3/T4 uptake is used to assess the binding capacity of the serum for thyroid hormone. This is used to help determine if the total T4 is reflecting the free T4, or if abnormalities in binding capacity are responsible for changes in T4 values and thus this test is only useful in conjunction with Total T4 or Total T3. In the Resin T3 Uptake test, labeled hormone is added to the patient's serum. If there is an increase in binding capacity, more labeled hormone will be bound to the binding proteins and thus less will be left free in the serum. The free labeled hormone in the serum is measured and usually reported as a percent of the total labeled hormone added. If a patient has a high total T4,
it may be due to overproduction of thyroid hormone (Hyperthyroidism) or to an excess of one of the thyroid binding proteins, usually thyroid binding globulin (TBG). If the high Total T4 is secondary to high TBG, the Resin T3 will be low, otherwise it will be normal or elevated. Another way of putting this is that if the Total T4 or Total T3 deviates from normal in one direction and the Resin T3 uptake deviates in the opposite direction, then the abnormality is due to changes in binding capacity, otherwise it is secondary to a true change in thyroid function (i.e. Hyper- or Hypothyroidism). For example, if the binding capacity is increased because of high estrogens, the free labeled hormone will be decreased and the Resin T3 uptake will be decreased.
https://www.excelmale.com/forum/showthread.php?3184-Thyroid-Tests-and-What-They-Mean
