Veterinary Physiology-III

FUNCTIONAL (METABOLIC) EFFECTS

• Thyroid hormone increases the metabolic rate and stimulates growth in young ones. It increases the rate of O₂ consumption (up to 200%) and heat production which is known is calorigenic effect.
• Thyroid activity increases following low environmental temperature, thus involved in thermo-regulation to increase internal heat production by non-shivering thermogenesis
• Thyroid hormone causes increased metabolic activities in almost all tissues of body except the brain, retina, spleen, testes and lungs.
• Thyroid hormone increases the size and number of mitochondria to increase the ATP production. It increases the activities of the respiratory enzymes and enzymes involved with glucose oxidation and gluconeogenis. It stimulates Na⁺‑K⁺ ATPase activities, favours cations transport and thus stimulates mitochondrial O₂ utilization.
• BMR is greatly increased by Thyroid hormone .
• Thyroid hormone increases cardiac output ensuring sufficient O₂ delivery to the tissues. It potentiates the actions of β ‑adrenergic recep­tors to catecholamines, thus stimulates the rate and force of con­traction of the heart.
• Thyroid hormone increases glucose absorption from the intestine and facilitates insulin-mediated glucose absorption by the cells and produces glycogenolysis.
• Thyroid hormone increases protein synthesis by stimulating mRNA synthesis and the rate of formation of proteins (enzymes) by ribosomes.
• It activates catabolic pathway and inhibits anabolic pathway. It enhances the actions of catecholamines which include non‑shivering thermogenisis, lipolytic, glycogenolytic, gluconeogenic and insulin secretion .
• Thyroid hormone potentiates the stimulatory effects of other hormones on metabolism by increasing the uptake of glucose by cells, enhanced glycolysis, gluconeogenesis, and absorption from gastro-intestinal tract lipolysis, ketogenesis and proteolysis.
• Thyroid hormone lowers blood cholesterol level due to increased removal by liver, mobilizes fat for energy, depletes fat stores of the body, increases free fatty acid levels in blood and enhances oxidation of fatty acids. One of the major metabolic results of thyroid deficien­cy is a marked increase in the serum cholesterol, phospholipids, triglycerides and decreased free fatty acid levels.
• The thyroid hormones influences nervous functions at all levels. Injection of thyroxine causes increased spontaneous electrical activity in the brain.
• Thyroxine enhances the speed and amplitude of reflexes, wakefulness, alertness to various stimuli, awareness of hunger, memory and learning capacity. It regulates creatine phosphate production and storage in the muscle tissue.
• Thyroid deficiency is a major factor for obesity and in­creased body weight.