Triiodothyronine (T3) and thyroxine (T4) which alsoare also called thyroid hormones, have a critical role forin the development of the brain and body of a fetus and a large number of metabolic functions in cardiovascular, nervous, immune, as well asand reproductive systems of adults (Feldman et al., 2014). These hormones derived from tyrosine amino acid are mainly circulated by thyroid-binding proteins in the blood but, the littlea small amount (less than 1%) of them are transported from thyroidthe thyroid gland to target tissues as unbound, which are called free T3 and T4. Although the biologically active form of thyroid hormones is T3, it hasthere is less production of it in the thyroid gland compared to T4. Thyroxine is the major form of thyroid hormones secreted by the thyroid gland and in blood circulation. However, it is finally metabolized to T3 in target tissues/cells since it is synthesized as a prohormone of T3 (Barrett et al., 2016). ThyroidThe thyroid stimulating hormone (TSH) is produced and secreted by the anterior pituitary gland to induce thyroidthe thyroid gland forto carry out the biosynthesis of thyroid hormones. If blood concentrations of thyroid hormonehormones are decreased, the pituitary gland is stimulated to release TSH, if. If thyroid hormone levels are increased, the TSH secretion is suppressed. Therefore, it has also a quite an importance in terms of the assessment of the activity of the thyroid gland (Feldman et al., 2014).