Metabolic effect of thyroid hormone is

Thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3), act as the primary regulators of metabolic rate in almost all nucleated cells. By binding to nuclear receptors, they modulate gene expression to enhance mitochondrial activity and fuel utilization. These hormones are essential for growth, development, and the maintenance of body temperature. They influence the turnover of carbohydrates, lipids, and proteins to sustain systemic energy requirements. Rationale: A. Increased BMR is a hallmark effect of thyroid hormones, achieved through the up-regulation of the Na+/K+-ATPase pump. This energy-intensive process consumes high amounts of ATP, thereby increasing the basal rate of oxygen consumption. A higher basal metabolic rate ensures that the body maintains the necessary kinetic energy for vital cellular functions. B. Increased protein synthesis occurs as T3 stimulates the transcription of numerous structural and functional proteins. In a physiological state, thyroid hormones promote a positive nitrogen balance essential for growth and tissue repair. However, it is important to note that at pathological, thyrotoxic levels, the effect may shift toward protein catabolism and muscle wasting. C. Increased glycolysis is part of the hormone's role in enhancing glucose turnover to provide immediate fuel for mitochondria. T3 increases the expression of glycolytic enzymes and facilitates glucose uptake in peripheral tissues. By accelerating the breakdown of glucose, the hormone ensures a steady supply of substrates for oxidative phosphorylation and energy production. D. Increased heat production, or thermogenesis, is a direct consequence of increased metabolic activity and uncoupled respiration. Thyroid hormones stimulate the expression of uncoupling proteins (UCPs) in mitochondria, which dissipate energy as heat. This effect is crucial for maintaining homeothermy and explains why patients with hyperthyroidism often exhibit significant heat intolerance. E. All the above is correct because thyroid hormones exert a pleiotropic effect on metabolism that encompasses all these actions. They simultaneously increase the basal metabolic rate, stimulate the production of new proteins, and accelerate carbohydrate catabolism. This integrated response ensures that the body’s energy production and heat generation are appropriately matched to physiological demands.