Definition
Triiodothyronine, also called T3, is one of the two most important hormones produced in the thyroid gland. T3 is the most effective hormone in the thyroid. In its biological activity, T3 exceeds the thyroid hormone tetraiodothyronine, the so-called T4, three to five times. The two iodine-containing thyroid hormones are produced from the protein thyroglobulin. T3 consists of a thyroglobulin with three iodine groups, while T4 consists of a thyroglobulin with four iodine groups.
Introduction
The thyroid gland produces the hormones T3 and T4 when stimulated by the hormone TSH from the brain, more precisely the pituitary gland. The thyroid hormones containing iodine increase the energy metabolism of the cells and promote the release of the hormones insulin and growth hormone. They also have an influence on the cardiovascular system.
The hormone T3 is bound in the blood plasma to >99% to plasma proteins, especially to the thyroxine-binding globulin. Only <1% of the hormone quantity is freely available in the blood. T3 has a plasma half-life of about 24 hours, which means that it is deactivated by the body relatively quickly.
Values/normal values of T3 hormone
Most T3 is bound to proteins in the blood, while less than 1% is present in free form as free T3 (fT3). The concentration of the hormone in the blood is subject to daily fluctuations. At night there is an increase and during the day there is a decrease of the hormone in the blood.
Since only the free hormone fT3 is effective and the bound T3 serves as a hormone store, in practice it is mainly the free (not bound to protein) T3, the fT3, that is measured. The concentration of the hormone in the blood is given in nanograms per deciliter and for free T3 in picograms per milliliter. The normal range for T3 is in a range of 67 – 163 ng/dl.
The normal parameters for fT3 are 2.6 – 5.1 pg/ml. In hypothyroidism, fT3 is less than 2.6 pg/mL, while in hyperthyroidism it is greater than 5.1 pg/mL. It is important to note that the normal values may differ in different laboratories, therefore the reference values given there should always be considered.
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