I am very thin and I have hypothyroidism too. I always wondered how that could be, and then I read somewhere that your thyroid is very important for digestion, and digestion is very important for assimilation, so if you're not digesting your food, your assimilation goes down. A Chinese lady once explained to me how cleaning up the liver can in turn make the thyroid work better, but I can't remember the specifics. But also as in the post above mine, low iron can cause low thyroid function too.
Do you know what your body temperature is?