Thyroid hormone calorigenic effects

Because most of the energy produced by cellular respiration eventually appears as heat, an increase in cellular respiration necessarily leads to an increase in heat production (i.e., to a thermogenic or calorigenic effect). Thus, to the degree that thyroid hormones control BMR, they also control thermogenesis (17). 

The effect of thyroid hormones on oxidative processes has long been considered to be their most fundamental action in adult mammals. In 1895, Magnus-Levy made the classic observation that respiratory exchange is depressed in hypothyroidism and elevated in hyperthyroidism. Many factors modify the effect of thyroid hormones in the intact animal, such as nutrition, other endocrine secretions, and the general health of the animal (cf. Boothby and Sandiford, 1924 Sundaram el al., 1956 Money, 1956) a consideration of these fa( tors is beyond the scope of this review. It should be noted that the thyroid hormones appear to exert no calorigenic activity in lower vertebrates (cf. Fromm and Reineke, 1956). 

The natural thyroid hormone, levorotatory thyroxine (LTJ, is primarily responsible for the elevation of the basal metabolic rate (BMR) as reflected in the increase in oxygen consumption. This feature of thyroxine is denoted as the calorigenic effect. Another important property of thyroxine is its influence on the heart. The intrinsic heart-rate was found to be higher in patients with hyperthyroidism and lower than normal in patients with hypoth5U oidism. It has been concluded that the differences in heart-rate between subjects with hyper- and hypothyroidism are mainly due to thyroxine acting directly on the heart and not to its potentiation of the action of catecholamines [174]. 

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