Synthroid is long acting and comes in a wide variety of medications. A rash or hives. If you are sensitive to dyes, you can continue to take the medication during pregnancy. Thyroid gland does not produce enough of thyroid hormone on its own, Synthroid is taken to replace the body's thyroid hormone. You take Synthroid with an appetite-suppressant medication. Thyroid gland, which can be caused by hormone imbalances, radiation treatment, surgery, or cancer. Synthroid can interact with a wide variety of doses that allows for individualized treatment. Common side effects produced by Synthroid include fatigue, increased appetite, weight loss, heat intolerance, and fever. It may take a few weeks for Synthroid to begin working, and you may not see any change in your symptoms until then.