What is Hypothyroidism?

Hypothyroidism is a clinical syndrome that develops due to a deficiency of thyroid hormones. According to the pathogenesis, hypothyroidism can be primary (due to the pathology of the thyroid gland itself) and secondary (due to TSH deficiency), with more than 99% of cases occurring in primary acquired hypothyroidism. The prevalence of hypothyroidism in the general population is about 2%, and in some age groups (elderly women) it can reach 6-8%.

Acquired Hypothyroidism
Acquired hypothyroidism is one of the most common endocrine diseases. The main reason for persistent primary hypothyroidism is chronic autoimmune thyroiditis, followed by iatrogenic hypothyroidism, which develops as a result of thyroid surgery or radioactive 131I therapy.

The epidemiology of hypothyroidism is largely determined by the fact that its most common cause is autoimmune thyroiditis. Firstly, autoimmune thyroiditis, like most other thyroid diseases, is 10 or more times more common in women. Secondly, this disease is characterized by a long course and leads to hypothyroidism after many years and decades from its onset. In this regard, hypothyroidism is most common among women over the age of 50-60. If the prevalence of hypothyroidism in the general population is about 1%, among women of childbearing age – 2%, then among women over 60 this indicator in some populations can reach 10-12% or more.

Causes of Hypothyroidism

Of greatest clinical importance is hypothyroidism, which developed as a result of chronic autoimmune thyroiditis, as well as iatrogenic hypothyroidism (postoperative, in the outcome of therapy with radioactive 131I). Iatrogenic hypothyroidism accounts for at least 1/3 of all cases of hypothyroidism. In these diseases, hypothyroidism in most cases is persistent irreversible. Along with this, for many diseases of the thyroid gland (destructive thyroiditis), as well as when exposed to a number of substances (large doses of iodine, thyreostatics), transient hypothyroidism can develop, which self-confines either during the natural course of these diseases, or upon termination of exposure to the factor that caused it (withdrawal thyreostatics). In some cases, the genesis of primary hypothyroidism remains unclear (idiopathic hypothyroidism).

One of the causes of primary hypothyroidism can be severe iodine deficiency. Mild and moderate iodine deficiency cannot lead to hypothyroidism in adults under normal conditions. In newborns, due to moderate, and sometimes even slight iodine deficiency, due to the combination of a low iodine content in the thyroid gland and a high level of thyroid hormone metabolism, transient neonatal hyperthyrotropinemia may develop. Pregnant women with iodine deficiency may develop relative gestational hypothyroxinemia. Nevertheless, the last two phenomena should not be fully identified with hypothyroidism syndrome.

The reason for the relatively rare secondary hypothyroidism, as a rule, is various destructive processes in the hypothalamic-pituitary region. Most often, we are talking about macroadenomas of the pituitary gland and suprasellar structures, as well as surgical interventions for these diseases.

Pathogenesis during Hypothyroidism

With a deficiency of thyroid hormones, changes develop in all organs and systems without exception. Since the main function of thyroid hormones is to maintain the main metabolism (cellular respiration), when they are deficient, there is a decrease in oxygen consumption by tissues, as well as a decrease in energy expenditure and utilization of energy substrates. For the same reason, with hypothyroidism, there is a decrease in the production of a number of energy-dependent cellular enzymes that ensure their normal functioning. The universal change that is found in severe hypothyroidism is mucinous edema (myxedema), the most pronounced in connective tissue structures. Myxedema develops due to excessive accumulation in the interstitial tissues of hyaluronic acid and other glycosaminoglycans, which, due to their hydrophilicity, retain excess water.