T3 (Triiodothyronine)

The follicular cells of the thyroid gland synthesize thyroid hormones:

• thyroxine T4
• triiodothyronine T3,

which are stored in association with thyroglobulin, the main protein of the follicle-filling colloid. Only 20% of the T3 present in the T3 in the blood is secreted directly by the thyroid gland. The remaining 80% is formed in peripheral tissues by deiodination of T4 to T3.

In normal conditions, thyroid hormone secretion is regulated on a feedback basis by hormones of the pituitary and the hypothalamus. Thyroglobulin (TRH) produced by the hypothalamus stimulates the secretion of thyrotroponin (TSH) by the pituitary, which in turn in turn stimulates the synthesis of thyroid hormones. Released into the blood thyroid hormones are transported to target tissues in mostly in combination with carrier proteins, mainly thyroxine-binding globulin thyroxine-binding globulin TBG), to a lesser extent with prealbumin and with albumin. Only 0.05% of T4 and 0.15% of T3 exist in the free form The free form of hormones is the biologically active fraction. Into cells Only free hormones can be transported. Therefore, it is believed that determination of FT3 and FT4 provides more information than determination of levels of total hormones. The diagnosis of thyroid dysfunction involves the determination of serum levels of TSH and free thyroxine FT4 and, less commonly, free triiodothyronine FT3.

Thyroid hormones affect the function of all tissues and organs. They are essential for normal growth, maturation and differentiation of cells, especially the CNS, increase thermogenesis and regulate lipid and carbohydrate metabolism.

An increase or decrease in the concentration of total thyroxine (TT4) is most often caused by hyperthyroidism or hypothyroidism. However, the concentration of this hormone can undergo changes despite the absence of thyroid disorders in patients with thyroxine-binding globulin (TBG) abnormalities An increase in TBG capacity means that despite a state of euthyroidism, the concentration of TT4 may be elevated, while a decrease in TBG capacity will in turn cause a decrease in TT4 concentration. Such factors affecting the results of TT4 determinations may be drugs or other disorders that directly affect the increase or decrease of thyroxine-binding globulins thyroxine (TBG), thereby inhibiting the binding of T4 by TBG.

The causes of the increase in TT3 and TT4 concentrations associated with the increase in concentrations of binding proteins (TBG):

• hyperestrogenism: pregnancy, estrogen use (HTZ, oral contraceptives)
• genetically determined disorders
• hypothyroidism
• acute infectious hepatitis
• drugs: clofibrate, opiate compounds, heroin, perphenazine, 5-fluorouracil, amiodarone

Reasons for the decline in TT3 and TT4 concentrations, associated with a decrease in the concentration of binding proteins (TBG):

• severe hypo- and dysproteinemias
• hormones and their derivatives (STH, glucocorticosteroids, androgens and anabolic steroids)
• genetically determined disorders
• hypothyroidism
• severe liver damage-especially cirrhosis
• nutrition
• Drugs: heparin, salicylates, L-asparaginase, diphenylhydantoin, penicillamine, sulfonylurea derivatives, phenytoin, phenylbutazone.