Inhibition of thyroid type 1 deiodinase activity by flavonoids

Some dietary flavonoids inhibit thyroperoxidase and hepatic deiodinase activity, indicating that these compounds could be classified as anti-thyroid agents. In this study, we evaluated the in vitro effect of various flavonoids on thyroid type 1 iodothyronine deiodinase activity (D1). D1 activity was measured in murine thyroid microsome fractions by the release of 125I from 125I-reverse T3. D1 activity was significantly inhibited by all the flavonoids tested; however, the inhibitory potencies on thyroid D1 activity differed greatly among them. A 50% inhibition of D1 activity (IC50) was obtained at 11 μm baicalein, 13 μm quercetin, 17 μm catechin, 55 μm morin, 68 μm rutin, 70 μm fisetin, 72 μm kaempferol and 77 μm biochanin A. Our data reinforce the concept that dietary flavonoids might behave as antithyroid agents, and possibly their chronic consumption could alter thyroid function.