Faith, you would be surprised to see how many people with Hypothyroidism are actually underweight -ie catabolic state. I am one and I researched the topic, it happens.
If you are gonna use a herb, I do reccomend maca. Helps with libido, mood, mind and energy (a bit on everything but especially libido). I think it has an stabilizing effect on hormones, but from what I know it benefits women more.