Formation of thyroid hormone
The thyroid gland is a bilobed organ in the anterior portion of the neck. Thyroid gland in normal adult weighs 20-25 grams. Thyroxine the hormone is secreted by this gland. Thyroxine is stored in the colloid of the thyroid follicles, a form of glycoprotein called thyroglobulin.
Hydrolysis of thyoglobulin yields monoiodotyrosine, diiodotyrosine, triiodotyrosine and thyroxine. Of these, triiodothyroxine is considered to possess a biological potency greater than thyroxine.
Thyroxine is synthesised in thyroid gland from tyrosine. First inorganic iodide is oxidised to organic iodide (2I→I2).Tyrosine is iodinated in the third position to form 3 - monoiodotyrosine. The next iodination occurs in the fifth position to form 3,5-diiodotyrosine.
Two molecules of diiodotyrosine couple to form a mole of tetraiodotyrosine which is thyroxine. Alanine is liberated.
Synthesis of thyroxine is accelerated by thyroid stimulating hormone (TSH) and inhibited by antithyroid drugs like thiocarbamides and aminobenzenes. Thyroid has the capacity of trapping inorganic iodine from circulation and storing it for utilization in the synthesis of thyroxine and its precursors. Depending upon the need for thyroxine and its iodinated derivatives, a proteolytic enzyme hydrolyses thyroglobulin, under the stimulating influence of TSH.