T3 is mainly responsible for the actions of thyroid hormone in the various target organs.
Most of the hormone T3 is formed extra thyroidal, especially in the liver by enzymatic deionization in position 5 ‘of T4. For this reason serum concentrations of T3 reflect more functional state of peripheral tissues than the thyroid gland secretory performance. Conversion of T4 to T3 generates reduction decreases in serum T3. Conversion is diminished by drugs (propranolol, corticosteroids, amiodarone) or in terms of non-thyroid disorders severe (‘low T3 syndrome “).
As T4, more than 99% of the amount of T3 is connected to the carrier protein, but at a 10 times lower affinity. T3 is more metabolically active than T4, but its effect is less extended.
Recommendations for determining T3 – T3 thyrotoxicosis diagnosis (suppressed TSH with normal T4) or where the FT4 is normal in the presence of signs of hyperthyroidism; determining prognosis in patients with Graves’ disease; evaluation of amiodarone-induced thyrotoxicosis; evaluation of artificial thyrotoxicosis (induced Cytomel); T4 replacement therapy monitoring.
Patient preparation – fasting :
Reference values - are based on age:
Pregnant women have higher concentrations of T3 correlated with gestational age:
– First quarter: 1.61-3.53 nmol / L;
– Second quarter: 1.98-4.03 nmol / L;
– Third quarter: 2.08-4.02 nmol / L.
Conversion factors: nmol / L x 0.651 = ng / mL; ng / mL x 1,536 = nmol / L.
Clinically low values – <0.77 nmol / L; high level:> 4.62 nmol / L.