I love green tea, but since I became ill with thyroid disease I have been unable to tolerate it. I get an allergic type reaction -- sneezing, rhinitis & itching all over. Probably due to the high level of histamines (as in all teas). It also acts as a diuretic for me, and makes me go to the bathroom every half hour !
Green tea contains L-thiamine, which is a mood balancing substance, so in theory it ought to be good for thyroid! I have not heard of it being harmful for the thyroid.