The purpose of this research study is to measure the changes in energy metabolism (how the
body burns energy), cardiovascular function (heart function), and lipid metabolism
(cholesterol break down and building) before and after thyroidectomy (surgical removal of
thyroid gland) in response to two approved therapies for hypothyroidism: levothyroxine (LT4)
or Liothyronine/levothyroxine (LT3/LT4) combination therapy.