How thyroid hormone helps in growth and melamorphosis is not clear -Thyroxine accelerates both synthesis

omes very small, as small as that of a domestic fly. How thyroid hormone helps in growth and melamorphosis is not clear -Thyroxine accelerates both synthesis and degradation of proteins, because all the metabolic activities are enhanced by. thyroxine. But it is now known that, at optimal dose, thyroxine causes a +ve nitrogen balance in the young, that is, growth ensures. Further, regarding acceleration of metamorphosis, the synthesis of enzymes that cause dissolution o f tail of the -tadpole is accelerated. It is possible that development of lung etc. during the metamorphosis of tadpole, is largely due to syn-thesis of proteins (including enzymes) required for these organ development and the protein synthesis is due to the thyroxine. Metabolism The thyroid hormone has pronounced effects on metabolism. They are briefly mentioned below: (i) General metabolism. Thyroxine increases general metabolism; the basal metabolic rate (BMR) increases, the subject loses weight' and the body temperature rises Before the introduction of RIA, the BMR used to be routinely estimated for diagnosis and1 determining the efficacy of the treatment in thyroid disorders The over production of heat (calories), that is, calorigenesisi (under the influence of thyroxme), does not occur in all organs Thus, heart, liver, skeletal muscles and kidney show great' calorigenesis under the influence of thyroxme, but brain and gonads (testis/ovary) are not affected The cause of I calorigenesis by thyroxme is still a mystery In the 1960s it was generally believed, to be due to 'uncoupling of oxidative phosphorylation', (for explanation of the term uncoupling of oxidative phosphorylation, see chap 2, sec VII, 'breakdown and> resynthesis of ATP The ATP cycle) This theory is now discarded In recent years, another theory, with strong experimental, data and arguments, have been introduced the thyroid hormones cause increase of Na+ K+ ATPase activity Recall, the job -of the Na+ K+ATPase is to enhance pumping out of the Na+from interior of the cell and draw in of K+ (from ECF) to interior! of the cell As this pumping out (of Na+) and in (of K+) require energy, thyroxme causes calorigenesis It has been estimated, that some 30 to 40% of total calor-igenesis is accounted by ealorigenesis of heart alone (n) Fat metabolism ln hyperthyroidism, serum cholesterol concentration is low whereas the same becomes high in hypothyroidism Thyroxme, because of the fact that it enhances enzyme activity, accelerates both synthesis and catabohsm of cholesterol Therefore in' hypothyroidism both are reduced but the removal (catabohsm) of cholesterol suffers more than its synthesis, resulting in accumulation of cholesterol Thyroxme can enhance the hpolytic action of adrenalin, so that plasma FFA concentration rises,-although thyroxme does not directly cause hpolysis (in) Carbohydrate metabolism Thyroxme stimulates neoglucogenesis and. glycogenolysis These tend to increase blood sugar level Also the glucose absorption from the gut is accelerated However, thyroxme also increases peripheral utilization (catabohsm) of glucose, which should lower the blood glucose level The outcome depends on the resultant of these opposing actions Generally, however, thyroxme increases blood sugar level Development of thyrotoxicosis in a patient of diabetes melhtus is always alarming It is believed that the thyroid hormonesi (TH) do not have a direct effect on carbohydrate, some of the effects of TH on carbohydrate metabolism are due to the facti that TH cause enhancement of actions of such hormones like adrenalin (which powerfully increases glycogenolysis) or insulin, (which powerfully increases the peripheral utilization of glucose) The calorigenesis, due to the TH, is a big factor for, increased peripheral utilization of glucose Mucopolysaccharide metabolism deserves special mention Adult full blown’ hypothyroidism is called my x edema, because W M Ord, who coined the name myxedema in 1878, thought that, in adult i hypothyroidism, excess mucus accumulated underneath the skin to present the appearance of edema The edema is actually, due to accumulation of hyaluronic acid and protein which in turn causes local water retention (iv) Protein metabolism -Thyroxme causes stimulation of both synthesis (anabohsm) and degradation (catabohsm) of proteins because both sets of> enzymes are stimulated by thyroxme The actual outcome is the resultant of these two opposing effects However, when the; amount of the thyroid hormones is optimal, then protein anabohsm dominates the picture (v) Mineral metabolism Thyroxme,. causes loss of calcium via urine In hyperthyroidism, calcium loss may result in osteoporosis Thyroxme causes increased, activity of the Na+ K+ pump system of the cell On individual systems (i) Cardiovascular system In patients suffering from hyperthyroidism, tachycardia, increased cardiac output, high systolic blood pressure, and increased pulse pressure are common All these features are remimscient of increased adrenergic activity It is known, that thyroxme causes an increase! in p adrenergic receptor population,so that in hyperthyroid states, increased b adrenergic activity is seen In addition, thyroxme is required for the normal degree of contractility of the myocardium Cause of overactivity of heart (tachycardia, m-> creased contractility, increased cardiac output), as stated above, is generally believed to be due to increased population of b. adrenergic receptors (causing increased effect of adrenalin's action) Some recent works, however hi