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.