Symptoms of hyperthyroidism arise from the elevated levels of thyroid hormones produced by the overactive thyroid gland. Excessive intake of thyroid hormones can produce these symptoms as well. Characteristic symptoms include a rapid heart rate, excessive sweating, intolerance to heat, tremor, nervousness, or agitation. Other symptoms can include fatigue, weight loss, hair loss, increased appetite, problems with concentration, frequent bowel movements, and irregular or decreased menstrual blood flow in women. Sometimes, thyroid nodules or an enlarged thyroid gland (goiter) are apparent. With severe hyperthyroidism, cardiac arrhythmias, fever, high blood pressure, and heart failure can develop.