**1. Introduction**

Hyperthyroidism (hyperthyreosis) is the condition that occurs due to excessive synthesis and release of the thyroid hormones T3 (triiodothyronine) and/or T4 (thyroxine) by thyrocytes and as a result of hyperstimulation of cells having receptors specific to these hormones.

A negative feedback blocks the secretion of thyrotropin (TSH) from the pituitary gland. Subclinical hyperthyroidism is characterized by normal levels of free thyroxine (f T4) and free triiodothyronine (f T3), with TSH below the reference range in the blood.

The term "thyrotoxicosis" refers to the clinical syndrome associated with the increased level of thyroid hormones caused by their enhanced production by the thyroid gland, their excessive release resulting from the gland destruction (e.g. the Hashitoxicosis phase in chronic thyroidits of Hashimoto's type), as well as due to the exogenous supply of thyroid hormone preparations in an abnormal, too large dose, and casual poisoning with thyroid hormone preparations (thyrotoxicosis factitia). Thus, thyreotoxicosis refers not only to the level of hormones in the blood, but is also associated with the level of sensitive cells that undergo activation, which leads to the manifestation of subjective and objective symptoms.
