Hyperthyroidism creates excess thyroid hormone. Since the thyroid gland produces hormones that regulate the body's metabolism, not surprisingly, the disease increases the body's normal metabolic functions. This leads to weight loss, increased appetite, a fast or irregular heartbeat, sweating, nervousness, irritability, fatigue, and muscle weakness. Graves' disease, the leading cause of hyperthyroidism, is characterized by puffy or bulging eyes, light sensitivity, double vision, and increased tears. It can also lead to thick skin that feels hard to the touch. Goiter (an enlarged thyroid gland) is another way hyperthyroidism affects the body. It can make the neck appear swollen.