10-24-2007, 05:21 AM
the thyroidhormones T3 and T4 are transported in the body via proteins that bind them. Only free (unbound) T3 and T4 are active in the body. Measuring T3 uptake (the RT3U test)helps estimate the availability of thyroxin binding globulin (TBG) -- the protein that carries most of the T3 and T4 in the blood. RT3U is inversely proportional to TBG levels: that is, if the TBG level goes up, the RT3U values will go down. A higher RT3U value means less TBG is available, possibly as a result of hyperthyroidism.