T3 | Triiodothyronine

Triiodothyronine (T3) is essentially responsible for the development of the effects of thyroid hormones in various target organs. The concentration of T3 in the serum reflects not so much the secretory activity of the thyroid gland, but rather the functional state of the peripheral tissue. All T3 is produced from T4. The effect of triiodothyronine in the periphery is incomparably stronger than that of thyroxine (T4). When the conversion of T4 to T3 is impaired, the concentration of T3 decreases. This may occur due to the effects of medications, for example, propranolol, glucocorticoids, or amiodarone, and in cases of severe non-thyroidal illness. T3 determination is used in the diagnosis of T3 hyperthyroidism, identifying early stages of increased thyroid function, and confirming the diagnosis of actual thyrotoxicosis.