organification of extra .amount ofiodine does not occur The whole phenomenon is called

me. the iodine trapping mechanism becomes less efficient, and organification of extra .amount ofiodine does not occur The whole phenomenon is called auto re gulation of the thyroid gland In this way, the availafability of iodine for thyroxine synthesis remains constant even in the face of wide fluctuation of food iodine content Mechanism of auto regulation may be as follows less iodine makes The thyroid gland more sensitive to the TSH (recall, the TSH helps in iodinetrappping) and vice versa Exposure to cold and thyroid secretion are int err elated, specially in the new born When exposed tothe cold external world, the freshly bom baby shows a rise of TH secretion. It appears that the stimulus of cold acts on the hypothalamus -- increased secretion of TRH rise of TSH rise of T4. Hypothalamus - piturtary - thyroid axis can now be underswood. These three structures (viz hypo-pit_thy) act in a concert and taken together they form an axis. A break down, anywhere in the axis causes a break down of thyroid regulation and causes signs and symptoms (disease). DEGRADATION OF T4 AND T3 Some 20 to 30% of the total thyroid hormones (T4 and T3) are conjugated in the liver usually with glucuronic acid, and is exreted with bile and undergoes enterohepatic circulation. Rest of the hormone (i. e . about 70%) is deaminated. The deaminated products are tetraiodothyro acetic acid and triiodothyroacetic acid, [abbreviated, tetrac and triac respectively] They undergo still further changes and then finally metabolically disposed. APPLIED PHYSIOLOGY Hyp of unction Two forms of hypofunction of the thyroid are known, viz. cretinism and adult hypathyroidism. The clinical Fig. 6.2.5. Summarised view of signs and symptoms of cretinism features are described here,the description of laboratory features has been given later Cretinism it hypothyroidism in the young Clinical features, in a full blown cast, are. retardation of growth of all forms, namely, physical, mental and sexual As a result the child is dwarf, pot bellied (because the muscles of abdomen are weak but abdominal visceral growth not severely restricted) and mentally idiot or imbecile