Hyperthyroidism (overactive thyroid) occurswhen the thyroid gland produces too much of the hormone thyroxine. Hyperthyroidism can accelerate body’s metabolism, causing unintentional weight loss and a rapid or irregular heartbeat.Problems with the thyroid can be caused byiodine deficiency,autoimmune diseases, in which the immune system attacks the thyroid, leading either to hyperthyroidism (caused by Graves’ disease) or hypothyroidism (caused by Hashimoto’s disease) inflammation which may or may not cause pain.Both hypo- and hyperthyroidism can be dangerous, and if left untreated, hypothyroidism can lead to unconsciousness and death. On the other hand, hyperthyroidism can cause significant weight loss, infertility, a heart irregularity called atrial fibrillation and double-vision problems. The signs and symptoms of hypothyroidism vary, depending on the severity of the hormone deficiency. Problems tend to develop slowly, often over several years. At first, one may barely notice the symptoms of hypothyroidism, such as fatigue and weight gain. Or you may simply attribute them to getting older. But as the metabolism continues to slow, may develop more-obvious problems.