Flavonoids and thyroid disease

The most potent natural plant-derived compounds that can affect thyroid function, thyroid hormone secretion and availability to tissues is the group of flavonoids, i.e. plant pigments. They are present in our daily food, such as vegetables, fruits, grains, nuts, wine, and tea. Epidemiological studies suggest beneficial effects on health of flavonoids, which are commonly attributed to their activity as antioxidants. Experimental studies in vitro, however, showed inhibition of organification in thyroid cells and follicles by several flavonoids. Studies in vivo and vitro with synthetic and natural flavonoids showed displacement of T4 from transthyretin leading to disturbances in thyroid hormone availability in tissues. Radioactive labeled flavonoids appeared to be eliminated rapidly from the body mainly through excretion in the feces. In pregnant rats synthetic flavonoids cross the placenta and accumulate in the fetal compartment, including the fetal brain. Therefore, a high intake of flavonoids is contraindicated. In conclusion: flavonoids show strong interference with many aspects of thyroid hormone synthesis and availability.