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Assays for FT4, total triiodothyronine (T3), and free triiodothyronine (FT3) have largely supplanted measurements of total T4, resin T3 uptake (RT3U), and free thyroxine index (FT4I). It is particularly important to determine “free” serum levels (FT4 and FT3) in conditions associated with high circulating levels of thyroxine-binding globulin (TBG), such as during therapy with oral estrogen. Ultrasensitive assays for serum TSH have largely replaced older TSH assays. Table 26–2 shows the appropriate use of thyroid tests.