Iv`e been diagnosed with an underactive thyroid some yaers ago for which I`m now being treated. I tested negative for thyroid antibodies, so I`m still racking my brains as to what caused it. When I was young I always suspected that my oestrogen levels were higher than they should have been, though my doctor wouldn`t do tests. Iv`e read that excess oestrogen can damage the thyroid. Could this be to blame for my thyroid condition?