Unlike the thyroid hormone thyroxine (T4), most T3 is converted enzymatically from T4 in the tissues rather than being produced directly by the thyroid gland (see monograph titled “Thyroxine, Total”). Approximately one-third of T4 is converted to T3. Most T3 in the serum (99.97%) is bound to thyroxine-binding globulin (TBG), prealbumin, and albumin. The remainder (0.03%) circulates as unbound or free T3, which is the physiologically active form. Levels of free T3 are proportional to levels of total T3. The advantage of measuring free T3 instead of total T3 is that, unlike total T3 measurements, free T3 levels are not affected by fluctuations in TBG levels. T3 is four to five times more biologically potent than T4. This hormone, along with T4, is responsible for maintaining a euthyroid state. Free T3 measurements are rarely required, but they are indicated in the diagnosis of T3 toxicosis and when certain drugs are being administered that interfere with the conversion of T4 to T3.
Triiodothyronine, Free has been found in Davis's Lab & Diagnostic Tests
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