Patients who take Synthroid to treat an underactive thyroid need to be cautious that they do not take too much. Most patients on thyroid medicine have Hashimoto’s thyroiditis like you, while a smaller number take thyroid medicine because of thyroid cancer or Graves disease. Too much thyroid medicine increases the activity of the osteoclasts (the cells that break down bone) and may increase your risk for fracture. To find the correct dose, your health care provider will obtain a thyroid-stimulating hormone (TSH) blood test and once you are on the correct dose, monitor your levels periodically.