Triiodothyronine free (T3 free)

Why this test?

• To help evaluate the functioning of the thyroid gland;
• To diagnose thyroid disease;
• To monitor the effectiveness of thyroid treatment

Synthesis, secretion and action of thyroid hormones are controlled by the hypothalamic-pituitary-thyroid system. Thyrotropin-releasing factor (TRF), which is secreted by the hypothalamus, stimulates the synthesis and secretion of thyrotropin or thyroid-stimulating hormone (TSH). TSH stimulates the synthesis, accumulation, release and metabolism of thyroxine (T4) and triiodothyronine (T3).

T3 is considered the most biologically active thyroid hormone. As a result of peripheral deiodination of thyroxine, about 80% of circulating T3 is formed and only 20% is secreted in the thyroid gland itself. T3 and T4 circulate in bound to thyroxine-binding globulin (TBG), thyroxine-binding prealbumin, and albumin. Free T3, found in equilibrium between free and bound forms, is 0.2 - 0.4% of circulating, this figure for total T4 is about 0.03%. The shift in the fractions of these three hormones reflects the functional state of the thyroid gland.

Thyroid hormones T3-free and T4-free affect all stages of carbohydrate metabolism, regulate normal growth and development, maintain body temperature and stimulate heat exchange, and also participate in the metabolism of lipids and vitamins. Thyroid hormones are necessary for the harmonious development of the fetus in the intrauterine period and the child in the neonatal period.
The concentration of free T3 correlates with the concentration of total T3 at a normal concentration of thyroid-binding proteins. Measurement of free triiodothyronine concentrations is appropriate when the concentration of total T3 changes as a result of changes in the concentrations of thyroid-binding proteins, especially with changes in TBG content or low albumin concentrations. An isolated increase in the values of free T3 (T3 - toxicosis) accounts for about 5% of cases of hyperthyroidism.

An increase in the concentration of free T3 in the blood can cause diseases unrelated to the metabolism of thyroid hormones. It is necessary to carefully diagnose the thyroid gland in patients with diseases unrelated to the thyroid gland itself. For example, a decrease in the concentration of free T3 can be caused by anticonvulsant drug therapy (in particular, phenytoin) as a result of increasing its metabolism in the liver, and also indirectly as a result of impaired binding of the hormone to binding sites. Salicylates and phenylbutazone are anti-inflammatory drugs that also affect the binding sites of the hormone, but their effect on free T3 has not been fully determined. An increase in free T3 values in patients receiving heparin may be associated with the release of non-esterified fatty acids (NFA), which in turn disturb the ratio between free and bound hormone.

In what cases is it prescribed?

If you have an abnormal thyroid-stimulating hormone (TSH) level, especially with a normal free thyroxine (T4) result and/or signs or symptoms of hyperthyroidism