There are many types of hypothyroidism and the causes are complex. The common causes of hypothyroidism in adults are as follows: Autoimmune damage: The most common cause is autoimmune thyroiditis, including Hashimoto’s thyroiditis, atrophic thyroiditis, and postpartum thyroiditis. Thyroid destruction: This includes secondary hypothyroidism due to thyroid surgery and iodine 131 treatment. The 10-year cumulative incidence of hypothyroidism during subtotal thyroidectomy and iodine 131 treatment for Graves’ disease is 40% and 40%-70%, respectively. Iodine overdose: Iodine overdose can cause hypothyroidism in people with underlying thyroid disease and can also induce and aggravate autoimmune thyroiditis. The incidence of iodine-containing drug amiodarone induced hypothyroidism is 5%-22%. Anti-thyroid drugs: Anti-thyroid drugs, such as lithium salts, thioureas, and imidazoles, can also cause secondary hypothyroidism if used improperly. In addition, a number of other systemic disorders may also lead to hypothyroidism, such as pituitary or hypothalamic hypofunction resulting in TRH or TSH deficiency, which can lead to central hypothyroidism. Decreased or defective thyroid hormone receptors in peripheral tissues, the presence of circulating antibodies to thyroid hormones or decreased conversion of peripheral T4 to T3 can lead to peripheral hypothyroidism. Therefore, hypothyroidism is not a single disease but a group of diseases characterized by deficient thyroid hormone secretion in common, each type having a different etiology and treated differently.