Free triiodothyronine (FT3)

General Information

Triiodothyronine (T3) Is a thyroid hormone, most of which circulates in a state bound to proteins - thyroxine-binding globulin and albumin.

About 3% of circulating triiodothyronine (T80) is produced by peripheral transformation of thyroxine (T4), while the remaining 20% ​​is produced by the thyroid gland itself.

It represents only 0,3% of the total amount of triiodothyronine in the blood Free triiodothyronine (FT3), which provides the entire range of metabolic activity. FT3 is almost 5 times more active than FT4. It increases heat production and oxygen consumption in all body tissues except brain tissue, spleen and testes; stimulates the synthesis of vitamin A in the liver; reduces the concentration of cholesterol and triglycerides in the blood; accelerates protein metabolism; increases the excretion of calcium in the urine; activates bone metabolism, but more - bone resorption; It has a positive chrono- and inotropic effect on the heart; Stimulates reticular formation and cortical processes in the central nervous system. The concentration of T3 depends on the concentration of TSH (thyreotropic hormone), so its determination is informative to assess the status of the thyroid gland when the level of TSH changes.

In thyrotoxicosis, as a rule, the concentration of T4 and T3 (total and free) in the blood is increased, but in a small number of patients with T3-toxicosis, only the level of T3 is increased. This is usually associated with iodine deficiency, when TSH levels can be suppressed by isolated hyperproduction of T3 with unchanged T4 concentrations.