Total T3
What it measures, and what changes the number.
Total T3 measures all the triiodothyronine in the blood, both the small fraction that is free and the large fraction bound to carrier proteins such as thyroxine binding globulin and albumin. T3 is the most active thyroid hormone, and much of it is produced by conversion from T4 in tissues.
It is read with TSH, Free T4 and Total T4, and sometimes with Free T3 or reverse T3, to build a picture of thyroid hormone production and conversion.
Because most T3 is protein bound, anything that changes carrier protein levels, such as pregnancy or oestrogen-containing medicines, changes Total T3 without necessarily changing the free hormone. It is measured by immunoassay or mass spectrometry, and methods can differ. Reports may list it as Triyodotironina (T3).
Often read alongside: TSH, Free T4, Total T4, Free T3, Reverse T3