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Cinnamon has several effects on energy metabolism. However, no data exist on the impact of cinnamon intake on thyroid hormone serum concentrations and action, since thyroid hormones (THs) play a major role in metabolism.Male rats were treated with cinnamon water extract (400 mg kg(-1) body weight, 25 days). Cinnamon supplementation resulted in a lower serum total T3 level accompanied by normal serum T4 and TSH levels. The cinnamon-treated rats did not exhibit significant differences in TSHβ subunit, TRβ or deiodinase type 2 mRNA expression in the pituitary. In the liver, cinnamon did not change the TRβ protein expression or the deiodinase type 1 mRNA expression, suggesting that there were no changes in T3 signaling or metabolism in this organ. However, mitochondrial GPDH, a target gene for T3 in the liver, exhibited no changes in mRNA expression, although its activity level was reduced by cinnamon. In the cardiac ventricle, T3 action was markedly reduced by cinnamon, as demonstrated by the lower TRα mRNA and protein levels, reduced SERCA2a and RyR2 and increased phospholamban mRNA expression.This study has revealed that TH action is a novel target of cinnamon, demonstrating impairment of T3 signaling in the cardiac ventricles. © 2015 Society of Chemical Industry.