Hey Barnmouse. I have an underactive thyroid, as well. Google “goitrogenic foods” for a full list of foods that can inhibit thyroid hormone production. Crucifers top the list. However, it seems (according to most sources) that cooking destroys the potentially harmful properties. So make sure you’re cooking your cruciferous veggies, eat some seaweed, take iodine…and give it time. Hormones are slooow.
I haven’t eaten meat in over 12 years and totally support your decision not to, by the way.