Thyroxine is the main hormone secreted by the thyroid gland; has an important role in the  hypothalamic-pituitary system to regulate the thyroid and influences the general metabolism. T4 results from the coupling of two molecules of 3,5-diiodotirosine, is linked to thyroglobulin and thyroid follicular cells remain,from where it is excreted under the action of TSH.

Most thyroxine (99%) circulates in the blood bound to proteins. As the concentration of serum proteins carriage is subject to exogenous and endogenous influences (eg, increases in pregnancy and after administration of oral contraceptives and decreases in nephrotic syndrome), their status should be considered when evaluating the T4.

Recommendations for determining T4 – hyper- and hypothyroidism diagnosis (primary or secondary); monitoring of TSH suppressive therapy.

Patient preparation – fasting (fasting); if the patient is being treated with lipid-lowering drugs containing thyroxine, of blood drawing for T4 determination will be done 4-6 weeks after its discontinuation.

Method: ELISA

Reference values ​​- based on age

In the task T4 grow at 40-60%, from 11 to 12 weeks due to increased TBG:

– First quarter: 94.4-191 nmol / L;

– Quarter II: 102-208 nmol / L;

– Third quarter: 89.5-202 nmol / L.

Conversion factor: nmol / L x 0.077688 = mg / dL; nmol / L x 0.77688 = mg / L; mg / dL x 12,872 = nmol / L.