In practice, in the vast majority of cases, primary hypothyroidism occurs. . It is established that the most frequent cause of its development is autoimmune thyroiditis. However, hypothyroidism may develop after surgery on the thyroid gland (postoperative hypothyroidism), when treating thyrostatics (medical hypothyroidism), after irradiation with iodine radioactive isotopes (post-radiation hypothyroidism) and with endemic goiter. In some cases, the disease may develop as a result of prolonged use of large doses of normal, non-radioactive iodine, for example, in the treatment with iodine-containing antiarrhythmic amiodarone. The emergence of hypothyroidism is possible with tumors of the thyroid gland. A great rarity is hypothyroidism, which has developed in the outcome of subacute, fibrosing and specific thyroiditis. In some cases, the genesis of the disease remains unclear (idiopathic hypothyroidism).
Secondary and tertiary forms of hypothyroidism (the so-called central hypothyroidism) are associated with damage to the hypothalamic-pituitary system in diseases such as pituitary adenomas and other tumors of the sellar area, empty Turkish saddle syndrome, heart attacks and pituitary necrosis (their development is possible with DIC and massive bleeding). Etiological factors can also be inflammatory diseases of the brain (meningitis, encephalitis, etc.), surgical and radiation effects on the pituitary gland. The decrease in the functional activity of the thyroid gland in the central forms of hypothyroidism is associated with a deficiency of thyroid stimulating hormone (TSH). A lack of TSH can be isolated, but more often it is combined with a violation of the secretion of other pituitary tropic hormones (in such cases, hypopituitarism is indicated).
In addition to the acquired forms of hypothyroidism, there are congenital forms of the disease. The frequency of congenital hypothyroidism in Russia is on average 1 case per 4000 newborns. Causes of congenital hypothyroidism can be: aplasia and thyroid dysplasia, genetically determined defects in thyroid hormone biosynthesis, severe iodine deficiency, autoimmune thyroid disease in the mother (due to the penetration of thyroid blocking antibodies through the placenta), treatment of thyrotoxicosis in the mother thyrothyrosytorestoherapists. Among the rare causes include congenital TSH deficiency, as well as the syndrome of peripheral resistance to thyroid hormones.
The variety of reasons for the development of hypothyroidism is reflected in the etiopathogenetic classification proposed by the staff of the Endocrinological Research Center of the Russian Academy of Medical Sciences.