What is the most common side effect of Synthroid?

Common Synthroid (levothyroxine) side effects include excessive sweating, trouble sleeping or insomnia, fast heart rate, headache, irritability, increased appetite, weight loss, heat intolerance, hair loss, diarrhea. Rare but serious Synthroid side effects include tremors, mood changes, difficulty breathing or shortness of breath, decreased bone mineral density, harder-to-manage diabetes, chest pain, and seizures. Side effects like these are more likely to happen when your dose is too high. They may mimic symptoms of hyperthyroidism — the opposite issue of hypothyroidism. Your prescriber will check your thyroid hormone levels regularly as a result.