Role of the hypothalamic–pituitary–thyroid axis in metabolic ...
Metabolic physiology of thyroid
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Transcript of Metabolic physiology of thyroid
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The role of thyroid hormones in the regulation of metabolism.
By….S.Hari Baskar
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THYROID GLAND
• Thyroid gland synthesizes two major of hormones: Thyroxine and Triiodothyronine
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Functions of iodine containing hormones
Necessary for normal growth, differentiation, sexual and mental development
Regulate the rate of metabolism
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• Required for Growth Hormone action
• Increases mitochondrial oxidative phosphorylation
• Increases activity of adrenal medulla
• Induces enzyme synthesis
• Result:Stimulation of growth of tissues and increased metabolic rate along with Increased heat production
Anabolic Actions of Thyroid Hormone
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• About 1/3rd of all ATP in the body is used by the Na+/K+ ATPase
• T3 increases the synthesis of Na+/K+ pumps, markedly increasing ATP consumption.
• T3 also acts on mitochondria to increase ATP synthesis
• The resulting increased metabolic rate increases thermogenesis (heat production).
One Major Target Gene of T3: The Na+/K+ ATPase Pump
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Calorigenic action
• Stimulates heat production in body secondary to increased oxygen consumption which increases BMR.
• Exception:- Adult brain ,Testis, Lymph nodes, Ovary, Uterus, Anterior pituitary.
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•In physiological concentration it is anabolic causing a positive nitrogen balance
Effect of thyroxine and triiodthyronine on the protein
metabolism:
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• In the increased concentration it is catabolic causing negative nitrogen balance
• Thus muscle weakness and cretinuria are common manifestations of a Hyperthyroid patient.
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Effect on carbohydrate metabolism
T3 and T4 leads to overall increase in enzymes causing
• Increased glucose absorption from GIT.
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• Acceleration of Glucose metabolism-Rapid uptake by cells, Enhanced glycolysis and increased insulin secretion and its effects on carbohydrate metabolism.
• Thus hyperthyroidism causes Diabetes mellitus
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Effect on Fat metabolism• Mobilization of fat from the
Adipose tissue.• Increase in level of free
fatty acid • Enhanced oxidation of free
fatty acids by cells of body.• Decrease in Quantity of
cholesterol, Phospolipids and TAG in plasma.
• All these factors leads to incresed loss of weight.
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• Decrease in plasma cholesterol is due to increased excretion in BILE.
• P.Chloesterol level drops even before increase in BMR, Showing that the action is independent of oxygen consumption.
• So, Hypothyroidism is associated with elevated plasma cholestrol levels, which can be reversed by thyroid administration.
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Vitamin metabolism• Thyroid hormones increase the
quantity of enzymes• Vitamins are essential for some
parts of enzymes and co-enzymes
• Thus, Thyroid hormones results in increased need of Vitamins.
• This is the reason for vitamin deficiency in Hyperthyroidism.
E.g. Excess thyroxin causes depletion of niacin leading to decrease in NADH concentration .
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On Water and Electrolyte balance
• Has a significant role in regulation of water and electrolyte balance.
• Impairment of thyroid function results in accumulation of Water and electrolytes which can be reversed by administration of these hormones.
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Permissive actions……
• T3 and T4 potentiate the stimulatory effects of Epinephrine, Nor-Epinephrine, Glucagon, Cortisol and Growth hormone on Gluconeogenesis, Lipolysis, Ketogenesis and Proteolysis of liable protein pool.
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On Sleep………• Due to exhausting effect
of thyroid hormone on the musculature an CNS, the hyperthyroid subject has a feeling of constant tiredness, But because of the excitable effects of thyroid hormone on the synapses, it is difficult to sleep.
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