Since THYROID is made to replace thyroid hormones, the most common side effects generally mimic an underactive or overactive thyroid (hypothyroidism or hyperthyroidism, respectively). Adverse reactions other than those indicative of hyperthyroidism, either initially or during the maintenance phase, are rare. If you experience chest pain, increased pulse rate, palpitations, excessive sweating, heat intolerance, or nervousness, please consult your doctor immediately; your dose may need to be adjusted. Common side effects may include weight loss, tremors, headache, upset stomach, vomiting, diarrhea, stomach cramps, nervousness, irritability, insomnia, excessive sweating, increased appetite, fever, changes in menstrual cycle and sensitivity to heat. This is not a complete list of side effects and others may occur. Call your doctor for medical advice about additional side effects or any side effect that bothers you or that do not go away.