Here is a good article written by my old aunts doctor.
He is well published and very smart.


THYROID HORMONES NOT FOR WEIGHT LOSS

Gabe Mirkin, M.D.

Overweight people who want to reduce should ask their doctors to draw thyroid tests because lack of that hormone causes obesity, but they should never take thyroid pills unless they need them.

Taking thyroid pills when your thyroid gland is normal causes osteoporosis, softening your bones and make them more likely to break; makes your heart beat irregularly and rapidly, and increase your risk for forming clots and having a stroke. Doctors no longer depend on blood levels of thyroid hormones to tell them if you need thyroid pills; instead they measure TSH, the hormone that is produced by your brain to regulate your thyroid gland. If your TSH is high, you probably need thyroid pills, even if blood levels of thyroid hormones are normal. If your TSH is normal and you want to lose weight, exercise more and eat the foods that fill you up
without contributing a lot of calories.