Description
T3 circulated in the blood as an equilibrium mixture of free and protein bound hormone. T3 is bound to thyroxine binding
globulin (TBG), prealbumin and albumin. The binding of these proteins is such that only 0.2%-0.4% of the total T3 is present in
solution as unbound or free T3 (fT3). This free fraction represents the physiologically active thyroid hormone and is three to
four times more potent than T4. Further, the concentrations of the carrier proteins are altered in many clinical conditions, such
as pregnancy. In normal thyroid function as the concentrations of the carrier proteins alters, the total triiodothyronine level
changes so that the free triiodothyronine concentration remains constant. Thus, measurements of free triiodothyronine
concentrations correlate more reliably with clinical status than total triiodothyronine levels.
