Hypothyroidism is a systemic disease caused by inadequate synthesis and secretion of thyroid hormones or poor physiological effects of thyroid hormones. If hypothyroidism begins in the fetus or neonatal period, it is called Cretinism; in children before sexual development, it is called juvenile hypothyroidism; in adults, it is called adult hypothyroidism. Etiology 1. Primary hypothyroidism: mainly seen in: (1) congenital thyroid deficiency; (2) thyroid atrophy; (3) diffuse lymphocytic thyroiditis; (4) subacute thyroiditis; (5) after destructive thyroid treatment (radioactive iodine, surgery); (6) impaired thyroid hormone synthesis (congenital enzyme defects, iodine deficiency or iodine excess); (7) drug suppression; (8) infiltrative damage ( lymphoma, amyloidosis, etc.). 2. Secondary hypothyroidism: mainly seen in pituitary and hypothalamic diseases. 3. Peripheral hypothyroidism: rare, hereditary disease in families.