Triiodothyronine (T3) Resin Uptake (RUR)

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Subject: Triiodothyronine (T3) Resin Uptake (RUR)

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Definition

  • 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%.

Use

  • 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.

Interpretation

  • Decreases when binding protein increases (pregnancy)

  • Increases when binding protein decreases (hyperthyroidism)

Limitations

  • 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.

 
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