Triiodothyronine (T3) is a hormone synthesised and secreted from the thyroid gland and formed by peripheral deiodination of thyroxine (T4).T3 and T4 are secreted into the circulation in response to TSH and play an important role in regulating metabolism. The T3 and T4 secretion are regulated by a negative feedback mechanism involving the thyroid gland, pituitary gland and hypothalamus. In the circulation 99.7% of T3 is reversibly bound to transport proteins, primarily thyroxine-binding globulin (TBG) and to a lesser extent albumin and pre-albumin. The remaining T3 does not bind to transport proteins but is free in the circulation (FT3) Unbound T3 is metabolically active.
Free T3 levels correlate with T3 secretion and metabolism. In hypothyroidism and hyperthyroidism free T3 levels parallel changes in total T3 levels. However measuring free T3 is useful when altered levels of total T3 occur due to changes in T3 binding proteins especially TBG. TBG levels remain relatively constant in healthy individuals but certain conditions such as normal pregnancy and steroid therapy can alter these levels. In these conditions free T3 levels are unchanged while total T3 levels parallel the changes in TBG.
3.5 – 6.5 pmol/L
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