The following points highlight the eleven main effects of thyroid hormones. Some of the effects are: 1. Effects on Oxygen Consumption, Heat Production 2. Effects on Growth and Differentiation 3. Cardiovascular Effect 4. Hematopoietic Effects 5. Gastrointestinal Effect 6. Pulmonary Effect 7. Effects on Foetal Development 8. Effects on Carbohydrate Metabolism 9. Effects on Protein Metabolism and Others.
1. Effects on Oxygen Consumption, Heat Production:
Thyroid hormones specially T3 increases O2 consumption and heat production in all tissues except the brain, spleen and testis. This contributes to the increased basal metabolic rate. Thyroid hormones also decrease superoxide dismutase levels, resulting in increased superoxide anion free radical formation.
2. Effects on Growth and Differentiation:
Thyroid hormones act synergistically with the growth hormone and thus, promote nitrogen retention, protein synthesis, body growth and tissue differentiation. The hormones also stimulate increased bone turnover, increased bone resorption and to a lesser degree of bone formation.
Normal development of brain requires thyroid hormones, without these hormones there is decreased myelinogenesis and retarded axonal ramification.
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Thyroid hormones stimulate metamorphosis in amphibians such as tadpoles, by inducing some enzymes like hyaluronidase and lysosomal hydrolases that are involved in regression of tail, gill and carbomoyl-phosphate synthase I and arginase that are involved in switch-overing from ammonotelism of tadpole to urotelism in adult.
3. Cardiovascular Effect:
T3 stimulates cardiac muscle contractility and increases diastolic contraction of the heart. Thyroid hormones have marked positive inotropic and chronotropic effects on the heart that account for the increased cardiac output and heart rate in hyperthyroidism and the reverse in hypothyroidism.
4. Hematopoietic Effects:
In hyperthyroidism, increased thyroid hormones lead to increased production of erythropoietin and increased erythropoiesis. The hormones also increase 2, 3-diphosphoglycerate content of erythrocytes allowing increased O2 dissociation from haemoglobin and increasing O2 availability to tissues.
5. Gastrointestinal Effect:
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Thyroid hormones stimulate gut motility, which can result in increased motility and diarrhoea in hyperthyroidism and the reverse in hypothyroidism.
6. Pulmonary Effect:
Thyroid hormones maintain normal hypoxic and hypercapnic drive in the respiratory centre.
7. Effects on Foetal Development:
The thyroid and anterior pituitary TSH secretion begin to act in the human foetus at about 11 weeks. Thus, the foetus growth is largely dependent on its own thyroid secretion.
8. Effects on Carbohydrate Metabolism:
Thyroid hormones enhance intestinal absorption of glucose and hepatic glycogenolysis by enhancing the activity of glucose-6- phosphatase. The hormones also increase hepatic gluconeogenesis by enhancing the activities of pyruvate carboxylase and PEPCK.
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They increase glucose oxidation by activating some mitochondrial oxidoreductases. Thus, in general thyroid hormones are antagonistic to insulin; they raise blood sugar and decrease the sugar tolerance.
9. Effects on Protein Metabolism:
Thyroid hormones inhibit protein synthesis and increase blood amino acid level and urinary NPN in large doses. In moderate doses, it shows protein anabolic effects.
10. Effects on Lipid Metabolism:
Cholesterol synthesis and degradation are both increased by thyroid hormones. The hormones may stimulate lipogenesis by enhancing the activity of malic enzyme and ATP-citrate lyase enzymes. Lipolytic action of adrenaline may be potentiated by thyroid hormones through up regulation of β-adrenergic receptors on the adipocyte membrane and, thus, indirectly enhance blood free fatty acid level.
11. Endocrine Effect:
Thyroid hormones increase the metabolic turnover of many hormones. For example, the production rate of Cortisol will increase in the hyper- thyroid patients. Patients with hypothyroidism show increased prolactin level and impaired ovulation.