Thyroxine (3,5,3′,5′-tetraiodothyronine, L-thyroxine or T4) is a hormone synthesized and secreted by the thyroid gland and plays an important role in regulating metabolism. Secretion into the circulation is in response to the pituitary hormone TSH (thyroid stimulating hormone) and is regulated by a negative feedback mechanism involving the thyroid gland, pituitary gland, and hypothalamus.
In the circulation, 99.95% of T4 is reversibly bound to transport proteins, primarily thyroxin binding globulin (TBG) and to a lesser extent albumin and thyroxine-binding prealbumin (TBPA). The remaining T4 is not bound to transport proteins, but is free in the circulation. This unbound fraction, or free T4 (FT4), is both metabolically active and a precursor to Triiodothyronine (T3).
Free T4 levels correlate with T4 secretion and metabolism. In hypothyroidism and hyperthyroidism, FT4 levels parallel changes in total T4 levels. 4 Measuring free T4 is useful when altered levels of total T4 occur due to changes in T4 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 T4 levels are unchanged, while total T4 levels parallel the changes in TBG.
10 – 21 pmol/L
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