The portion of triiodothyronine (T3) that is not bound and represents approximately 10% of circulating T3 in the blood. T3 is converted from T4 and the majority occurs in the liver and kidneys. The conversion process is dependent on Selenium and bile levels. Levels can be used to assess Hypo and hyper thyroid status, T4 to T4 conversion efficacy, Selenium sufficiency, and Iodine sufficiency.

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