Overactive and underactive thyroid glands can cause a wide range of symptoms, which are commonly called “thyroid disorders.” Both types of thyroid disorders lead to an enlargement of the gland. Symptoms of overactive thyroid include irregular heartbeat and heart palpitations and an elevated heart rate, heat sensitivity, unexplained weight loss, sweating and bulging eyes. Underactive thyroid disorders have many opposite symptoms, including a low heart rate, sensitivity to cold, unexplained weight gain and clammy skin. In both cases, additional complications will appear if the condition is left untreated. Thyroid disorders are caused by hormonal imbalances, and the conditions are typically managed through the use of hormone supplements.