I have hypothyroidism and take Levothyroxine (Synthroid) but I am thin as a rail. I inherited it from my mother, she is also on Levothyroxine but a much larger dose. Thyroid disorders are so more common lately because of everyday exposure to chemicals in our food, drinking water and environment which mimic thyroid hormones. Off the top of my head, Triclosan, an antibacterial agent in soaps, hand sanitizers, one particular brand of toothpaste (Colgate Total) and even I believe Microban plastic products is absorbed into the bloodstream and suppresses the thyroid gland. Chlorine and especially fluoride in tap water mimic thyroid hormone so I only drink bottled or reverse osmosis filtered water. Water pitcher filters only remove chlorine but not fluoride.

There is an entire class of man made and natural food source thyroid hormone disrupters called goitrogens that I also try to avoid. http://en.wikipedia.org/wiki/Goitrogen