as others have said, it could be for a variety of reasons.
i think that my main focus would be on general thyroid balancing, such as increasing iodine, etc, and considering going soy free and increasing overall fat intake. these tend to support thyroid function.
if you find that this doesn't help, consider including natural soy products (tempeh, tofu, miso, edamame) in moderation. this may also help boost thyroid/estrogen function.
there are other things you can do for thyroid function (yoga poses, washing practices using warm or cold water depending upon the thyroid process, etc), but i think that this stuff would make a good start.
i don't think that veganism is the issue per se. it can be, depending upon how it's "formulated" but it might also just be what it is.