The T3 uptake test is an older test used to determine the thyroid binding capacity available in the blood. It must be measured alongside total T4 levels and is used to determine the free T4 index (FTI). T3 uptake is not used as a standalone test and has become somewhat obsolete since the development of assays that measure free T4 directly.
A low T3 uptake is associated with hypothyroidism when T4 is low but is associated with increased thyroid binding globulin (TBG) when T4 is high.
An elevated T3 uptake is associated with hyperthyroidism when T4 is high, but is associated with low TBG when T4 is low.
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