**1. Introduction**

276 Thyroid Hormone

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One of the most pronounced effects of Thyroid Hormone (TH, Triiodothyronine (T3), Tetraiodothyronine (T4)) is modulation of metabolic efficiency, energy expenditure and calorigenesis. Thus, hypothyroidism results in decreased energy expenditure and basal metabolic rate accompanied by weight gain and cold intolerance, while hyperthyroidism results in hypermetabolism, weight loss despite increased energy intake, intolerance to heat, loss of lean mass, bone resorption and tachycardia. TH role in modulating metabolic efficiency has been realized for over a century but its cellular mode-of-action remained to be resolved.
