metamorphosis Growth in human beings, which is intimately intermingled with, the development (and hence will be considered

there. (5) Release. When necessary, the apical portion of the follicular cell engulfs (= endocylosis) little thyroglobulin from the colloid of the acinus. The engulfed Ihyroglobulin now fuses with the lysosomes of the follicular cells and then becomes subjected lo proteolysis by a protease enzyme of the lysosome. As a result of the proteolysis, T4 and T3 (thyroxine and Iriiodolhyronine) molecules are released into the venous blood draining the thyroid. Both endocylosis and proleolysis are favored by TSH. ' For a short account of Na+ K+ ATPase, see chap. 1, sec. I, as well as chap 2, sec I. Note , therefore, that TSH of anterior pituitary helps the biosynthesis and release of thyroid hormones in almost every step. Food iodine Ultimately the source of iodine in our body, in the natural conditions, is sea food. Thus, sea fish and sea animals are great sources of food iodine. The Japanese (whose country is surrounded by sea and who take heavy quantities of sea fish) consume a very high quantity of iodine in their food, whereas people staying far away from the sea (the sub Himalayan regions, particularly Terrai regions of India) have very little iodine in their natural food. Therefore thyroid deficiency is very common in the sub Himalayan belt of India. However, in our body, arrangements exist, by which practically the entire quantity of the available iodine can be trapped when the food iodine is low in amount. However, if the daily iodine intake is less than 50 gms, (normal recommended minimal intake, 200 gms per day), then despite this mechanism, iodine deficiency and thyroid disorder develop i.e , the above mentioned protective mechanism fails when the food iodine is exlremely low. Conversely, if the food iodine content is high, Ihe efficiency of the iodine trap falls (see also 'autoregulation', later in this chapter). In iodine deficient areas intake of iodized table salt is advocated. The Government of India (GOI), set up a 'Nalional Goitre Control Programme' in the second Five Year Plan. Recently, with a view to increase the supply of iodine in the sub Himalayan zones of India, the GOI has banned the sale of ordinary table sail in these zones and is supplying iodized table salt to replace ordinary table salt. A recently introduced term, 'thyroid hormone economy', is popular amongst the clinicians. Iodine released from degraded thyroid hormones is stored in the body and utilised for thyroxine synthesis. Large number of drugs, containing iodine also contribute to the food iodine. Owing to increased intake of such drugs, daily intake of iodine has shot upto 1000 gms/day in an average Weslern world man. Thyroglobulin (mol wt about 650,000) is synthesized by the follicular cells of the thyroid. Iodine metabolism, synopsis Iodine is ingested as food iodine or in drug. As stated already, daily intake of iodine varies Iremendously but let us take the 'average' as 500 gms per day. The total content of iodine in plasma and tissue fluid (ECF) is about 250 gms. The thyroid conlains approximately 8000 gms. The amount of iodine as T4 and T3 in the tissues is about 600 gms. Urinary output per day is about 488 gms and loss via the stool is about 12 gms/day, so that daily intake = daily output. Iodine is filtered in the glomerular filtrate and is reabsorbed. The reabsorption, however, does not depend on CARRIAGE OF THE HORMONES In the plasma both T4 (thyroxine) and T3 (triiodothyronine) are present but most of this T3 is obtained from peripheral tissues (recall, in the peripheral tissues, T4 is converted into its active form, T3). T4 and T3 circulate mostly (99%) as bound with plasma proteins, only about 1% is free. However, only the free form is active, the bound form functioning as reserve. Binding proteins for T4 are principally -(i) TBG (Ihyroxine binding globulin, an globulin) which accounts for about 75% (ii) TBPA, thyroxine binding pre albumin also binds the T4 to some extent, (iii) Albumin can also bind T4. T3 is bound mainly by TBG and to certain extent by TBPA. All these binding proteins are synthesized by the liver. TBG is a glycoprotein with a molecular weight of 54 K (= 54 kilo dalton = 54,000) T4 : T3 concentration ratio in the blood is about 95: 5. The T4 is converted lo T3 in the peripheri; T3 is thus the active hormone and T4 can be viewed as a prohormone. Protein bound iodine (PBI). An indirect estimation of the amount of circulating T4 and T3 can be made by PBI estimation. All iodines which are bound with plasma proteins are part of either T4 or T3. Normal range of serum PBI values is 4 to 8 gms/100 ml. FUNCTIONS 'The functions of the thyroid hormones can be divided into three major groups: (1) On growth and development (as well as on metamorphosis in the lower animals). (2) Metabolic, and (3) On individual systems. These will now be elaborated in details Growth, development and metamorphosis Growth in human beings, which is intimately intermingled with, the development (and hence will be considered together) can be classified as follows : Thyroid hormones affect all the above mentioned par-ameters, but some of them more spectacularly and others less so. In thyroid deficient children (cretin, see also, applied physiology, later in this chapter) the (i) skeletal growth is poor, therefore the child remains short statured. It's (ii) sexual growth and malurity do not occur ('infantile sex') and is (iii) mentally very retarded ('imbecile'/'idiot) Metamorphosis in the tadpoles require thyroid hormones. When extra thyroxine is added to the water, (containing tadpoles').the metamorphosis is very accelerated, the tadpole becoming a frog in very short time. The size of the frog however becis