@teal Levothyroxine can cause a variety of side effects — some are common, others severe. Here are some of the side effects caused by levothyroxine- Weight gain, Weight loss, Headache, Changes in appetite, Fever, Excessive sweating, Menstrual changes, Sensitivity to heat, Hair loss, Leg cramps, and some severe side- effects are Shortness of breath, Irregular heartbeat, Hives, and Severe fatigue. The long-term effects of thyroid medication include weight fluctuations, sensitivity to heat, joint pain, changes to your menstrual cycle, and possibly even autoimmune dysfunction. Thanks!