Cinnamon intake reduces serum T3 level and modulates tissue-specific expression of thyroid hormone receptor and target genes in rats - PubMed
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.
Has anyone come across this study before? I don't have access to the full study but it would appear cinnamon lowers circulating levels of T3. Based on the keywords, I believe Ceylon cinnamon was used. The human equivalent dose, about 65mg per kg, is pretty high considering that it was an extract that was tested. Still, it might be prudent to use cinnamon sparingly given that it also contains the blood thinner coumarin and is known to affect blood sugar and cholesterol levels.