Hypothyroidism and hyperthyroidism are disorders of the thyroid gland that are characterized by decreased or increased production of thyroid hormones. There are two main hormones synthesized by the thyroid gland: triiodothyronine (T3) and tetraiodothyronine (T4). Their secretion is regulated by the level of thyrotropic hormone (TSH) secreted by the pituitary gland. Thyroid hormones strongly stimulate all metabolic processes and play a key role in the growth and development of the child. Thyroid disorders are the most common endocrinopathies in childhood. Disorders that cause hypothyroidism are more common and can be classified as primary or secondary and as congenital or acquired hypothyroidism. Congenital hypothyroidism is the most common, curable cause of mental retardation. In the developed world, neonatal screening for congenital hypothyroidism has been introduced to prevent neurodevelopmental disorders. Acquired hypothyroidism is the most common thyroid dysfunction in children and adolescents. It is most often caused by autoimmune thyroiditis (Hashimoto's thyroiditis). Slow growth in a child is usually the first sign of hypothyroidism and levothyroxine treatment should be started. Hyperthyroidism is a rare but very serious disorder in childhood. Graves' disease is the most common cause of hyperthyroidism in childhood. It is an autoimmune disorder in which thyrotropin receptors are stimulated by antibodies. Possible options in the treatment of hyperthyroidism are antithyroid drugs, radioactive iodine treatment and thyroidectomy.