T3 uptake
What it measures, and what changes the number.
T3 uptake, also called the thyroid hormone binding ratio, is an older test that does not measure the amount of T3 in the blood. Instead it estimates how many unoccupied binding sites are available on thyroid hormone carrier proteins, chiefly thyroxine binding globulin, by seeing how much added labelled hormone is taken up by a binding resin or reagent.
It is read with Total T4, and the two are combined to calculate the free thyroxine index, an indirect estimate of free T4. It may also be reported with TSH and Total T3.
Results move in the opposite direction to binding protein levels, so conditions and medicines that change those proteins affect the number. It may be expressed as a percentage or a ratio depending on the lab. Reports may list it as T3 UPTAKE.