Consuming too many flavonoids, including quercetin, can potentially disrupt thyroid function. Therefore, individuals with thyroid disorders should be cautious when adding quercetin and other flavonoids to their diet. However, unless you have been consuming excessive quantities of fruits and vegetables daily, it is unlikely that the dose of quercetin you have been taking is the sole cause of your thyroid problem. Nevertheless, everyone’s tolerance levels vary, and what may not be excessive for one person could be for another.