Do you mean hypothyroid (underactive thyroid)? Then it would be low thyroid hormone
, not sure of the exact name of the hormone. As far as I know there isn't anything you can take for herbs, foods, etc to help increase that level. However there are foods that can enhance goiter(enlarged) thyroid, and there is an association between goiter and thyroid function.
I know a little about this because I have multinodular goiter, and am symptomatic for hypothyroid, but my blood chemistry is normal for thyroid hormones, which basically means they don't do anything.
If you did have an underactive thyroid, eventually the typical treatment is to recieve the hormone in pill form. in this case, as a vegan, you would want the synthetic hormone, since the natural version is actually made from animal thyroid. Yuck, huh?