The thyroid is a gland located in the neck that regulates metabolism. Thyroid diseases refer to various conditions that affect the normal function of the thyroid gland. These include hyperthyroidism, in which the thyroid gland is overactive, hypothyroidism, in which it is underactive, and thyroid nodules. Hyperthyroidism can cause symptoms such as rapid heartbeat, excessive sweating, weight loss and irritability, while hypothyroidism can lead to symptoms such as fatigue, weight gain, depression and cold intolerance. Thyroid nodules are usually not cancerous, but sometimes they should be examined to determine the risk of cancer. Thyroid diseases are diagnosed with blood tests and imaging tests and can be treated with thyroid hormone replacement, medications or surgery. Treatment usually aims to restore normal levels of thyroid hormones.