Green tea catechins may act as anti-thyroid agents, and high doses of green tea extract could adversely affect thyroid function.

“Gaucho genic anti-thyroid all potential of green tea extract in relation to catechin and rats admittedly this is an animal study and these doses are probably a little bit higher than most people are going to get with daily ingestion of a small amount of green tea but as you'll see here 1.5 grand person 2.5 5% grams they found that they say taken together these results suggest the catechin present in green tea extract might behave as antah as an anti thyroid agent and possibly the consumption of green tea at high dose could alter thyroid function adversely”