Synthroid is long acting and comes in a wide variety of doses that allows for individualized treatment. A large difference in how you feel. Although this is rare. Common side effects produced by Synthroid include fatigue, increased appetite, weight loss, heat intolerance, and fever. Synthroid is long acting and comes in a wide variety of medications. Some hair, but this effect is usually temporary. The hypothyroid problem. Synthroid is the number one prescribed treatment for thyroid disease, in conditions where the thyroid gland does not produce enough of thyroid hormone on its own, Synthroid is taken to replace the body's thyroid hormone. Synthroid may cause seizures at the beginning of treatment, although this is rare.