PHYSIOLOGY Thyroxine

PHYSIOLOGY Thyroxine

The hormones tri-iodothyronine (T ) and /l.sc -thyroxine (T 3 bound to thyroglobulin within the colloid. Synthesis within the thyroglobulin complex is controlled by several enzymes, in distinct steps: /uni25CF trapping of inorganic iodide from the blood; /uni25CF oxidation of iodide to iodine; /uni25CF binding of iodine with tyrosine to form iodotyrosine; /uni25CF coupling of monoiodotyrosines and di-iodotyrosines to form T and T . 3 4 When hormones are required, the complex is resorbed into the cell and thyroglobulin is broken down. T and T are lib 3 4 erated and enter the blood, where they are bound to serum proteins: albumin, thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA). The small amount of hormone tha t remains free in the serum is biologically active. The metabolic e ff ects of the thyroid hormones are due to unbound free T and T (0.3% and 0.03% of the total 3 4 circulating hormones, respectively). T is the more important 3 physiological hormone and is also produced in the periphery by conversion from T . T is quick acting (within a few hours), 4 3 whereas T acts more slowly (4–14 days). 4 PHYSIOLOGY Thyroxine

The hormones tri-iodothyronine (T ) and /l.sc -thyroxine (T 3 bound to thyroglobulin within the colloid. Synthesis within the thyroglobulin complex is controlled by several enzymes, in distinct steps: /uni25CF trapping of inorganic iodide from the blood; /uni25CF oxidation of iodide to iodine; /uni25CF binding of iodine with tyrosine to form iodotyrosine; /uni25CF coupling of monoiodotyrosines and di-iodotyrosines to form T and T . 3 4 When hormones are required, the complex is resorbed into the cell and thyroglobulin is broken down. T and T are lib 3 4 erated and enter the blood, where they are bound to serum proteins: albumin, thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA). The small amount of hormone tha t remains free in the serum is biologically active. The metabolic e ff ects of the thyroid hormones are due to unbound free T and T (0.3% and 0.03% of the total 3 4 circulating hormones, respectively). T is the more important 3 physiological hormone and is also produced in the periphery by conversion from T . T is quick acting (within a few hours), 4 3 whereas T acts more slowly (4–14 days). 4