Common triiodothyronine (T3)

General Information

Common triiodothyronine (T3)Reflects the total concentration of triiodothyronine bound to free and plasma proteins.

Triiodothyronine (T3)Is a thyroid hormone, an iodinated derivative of the amino acid tyrosine.

Most of the T3 hormone is produced extrathyroidically, especially in the liver, as a result of the loss of 4 atom of iodine by T1 (T3 contains 3 atoms of iodine).

About 4% of thyroxine (T30) is metabolized to T3 and to "reverse" (inactive) T3 forms. The latter has no hormonal action and does not inhibit TSH secretion.

About 3% of triiodothyronine (T20) is produced by the thyroid gland under the influence of TSH, and 80% - as a result of deiodination of T4 in peripheral tissues (mainly in the liver and kidneys). Therefore, serum T3 concentration reflects the functional state of peripheral tissues rather than the secretory activity of the thyroid gland.

The hormonal activity of T3 is three times higher than that of T4. Free T3 accounts for 0.3% of total serum hormone levels. More than 3% of T80 is associated with thyroxine-binding globulin (TBG), an increase in the concentration of which causes an increase in serum total T3 and total T4 levels, initially leading to a short-term decrease in free T3 and free T4. The secretions of T3 and T4 then intensify and their total content increases until normal levels of free T3 and T4 are restored.

Thyroid hormones inhibit the secretion of TSH by the principle of negative feedback, affecting the thyrotropic cells of the adenohypophysis.

Study material: Venous blood