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Thyroid hormones have been implicated in the regulation of nuclear 3,5,3'-tri-iodothyronine (T₃) receptor binding capacity (B_max) but, despite numerous in vivo and in vitro studies, there is considerable controversy regarding their exact role. Since changes in thyroid status alter energy balance and hence may influence T₃ receptor numbers, the effects of chronic hypothyroidism and T₄ treatment have been studied in young pigs under conditions of controlled energy intake. Four groups of animals comprising a hypothyroid, a euthyroid and a hyperthyroid group, all on the same level of food intake, and a hyperthyroid group on twice the amount of food were used. After 3 weeks on the treatment regimes, both the hypothyroid animals on the same level of food intake and the hyperthyroid animals on twice the amount of food had significandy increased B_max values (97% and 137% higher respectively) compared with euthyroid controls. However, there was no difference between controls and the hyperthyroid animals on the same level of food intake. In a second study, the effects of short-term treatment of euthyroid animals with T₃ was investigated. Results showed that in two groups of controls that received intravenous saline, those on a higher food intake had higher B_max values (76% increase). Intravenous T₃ administration to animals on a low food intake did not change the receptor numbers. In none of the studies was there any change in the dissociation constant of the receptors as a result of different treatments. It is suggested that, at least in postnatal life, thyroid hormones per se have no significant effect on nuclear T₃ receptor numbers in skeletal muscle. Instead, changes in B_max in response to thyroid status may be secondary to alterations in energy balance induced by these hormones.

Journal of Endocrinology (1995) 144, 233–242

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