Hi Karen... Well I've got bipolar (II) and am thin (like size 2 or 4, 115-120, and 47, so no spring chicken). I've been a bit heavier at other times (highest was at first major depression (at 20, hit 140, interestingly enough only 10 lbs short of my full term pregnancy weight 9 years later, but with the depression I was living on chocolate covered donuts, so... ). It may play into it that when I'm depressed, I can hardly be arsed to eat. When I'm hypomanic I don't take the time... too busy with projects. Never had any thyroid issues.
Have read that some thyroid issues can _mimic_ depression symptoms though, but not that they are linked.