Storage form of thyroid hormone-

The synthesis and storage of thyroid hormones occurs between the follicular cells and the colloid. The storage form of thyroid hormone is thyroglobulin. Thyroglobulin is a large glycoprotein synthesized in the follicular cells and has a molecular weight of around 650 000 with about 140 tyrosine residues, depending on the form of thyroglobulin. Approximately one quaer of these residues is iodinated at the apical-colloid interface. Once iodinated, thyroglobulin is taken up into the colloid of the follicle where, still incorporated in the protein, a coupling reaction between pairs of iodinated tyrosine molecules occurs. The coupling of two tyrosine residues each iodinated at two positions (di-iodotyrosine, DIT) produces tetra-iodothyronine or thyroxine (T4 ) whilst the combination of DIT with mono-iodotyrosine (MIT) produces tri-iodothyronine (T3 ). Such coupling can occur within a single molecule of thyroglobulin or between dimerized molecules of the protein. This coupling is catalyzed by TPO. Thyroid hormones are stored in this state and are only released when the thyroglobulin molecule is taken back up into the follicular cells. Stimulated by TSH, thyroglobulin droplets are captured by the follicular cells by a process of pinocytosis. Fusion of the droplets with lysosomes results in hydrolysis of the thyroglobulin molecules and release of T3 and T4. Reference : page 2285 Harrison's Principles of Internal Medicine 19th edition