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Subject: Triiodothyronine (T3) Resin Uptake (RUR)
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Measures unoccupied binding sites on TBG. Not a measure of T3 concentration, which is assayed by other methods for diagnosis of T3 thyrotoxicosis. Now replaced by free T4 (see Table 16.77).
Normal range: median value of 40.0% (0.40) with a 95% nonparametric range of 32.0–48.4%.
Only with simultaneous measurement of serum T4 to calculate T7 in order to exclude the possibility that an increased total T4 is due to an increase in TBG.
RUR is inversely proportional to unsaturated hormone-binding sites.
Total T4 × RUR is proportional to free T4 and inversely proportional to TSH.
Decreases when binding protein increases (pregnancy)
Increases when binding protein decreases (hyperthyroidism)
Normal in pregnancy with hyperthyroidism, nontoxic goiter, and in use of certain drugs (e.g., mercurials, iodine)
In some cases of severe nonthyroid illness, RUR does not fully compensate and does not adjust the T4 into the normal range.