The AFIAS T3 test is a fluorescence immunoassay (FIA) for the quantitative determination of total triiodothyronine (total T3) in human serum or plasma. The test is used as an aid in the treatment and monitoring of thyroid disorders. For in vitro diagnostic use only.
3,5,3' Triiodothyronine (T3) is a thyroid hormone with a molecular weight of 651 daltons.
It circulates in the blood as an equilibrium mixture of free and protein-bound hormone. T3 is bound to thyroxine-binding globulin (TBG), prealbumin and albumin. The actual distribution of T3 between the binding proteins is controversial, with estimates ranging from 38-80% for TBG, 9-27% for prealbumin and 11-35% for albumin.
T3 plays an important role in maintaining normal (euthyroid) thyroid function. Measurements of T3 may therefore be a useful component in the diagnosis of certain thyroid dysfunctions. Most reports show that T3 levels can be used to clearly distinguish euthyroid and hyperthyroid states, but there is a less clear distinction between hypothyroid and euthyroid states. Total T3 measurements may be useful when hyperthyroidism is suspected and free T4 is normal. For example, one particular type of thyroid dysfunction is T3 thyrotoxicosis, which is associated with a decrease in serum thyroid-stimulating hormone (TSH), an elevated T3 level, normal T4, normal free T4 and normal to elevated in vitro uptake values.
T3 levels are affected by diseases that affect TBG concentrations. Slightly elevated T3 levels may occur in pregnancy or during estrogen therapy, decreased levels in severe illness, renal failure, myocardial infarction, alcohol addiction, inadequate nutrient intake, and during therapy with certain drugs such as dopamine, glucocorticoids, methimazole, propranolol, propylthiouracil, and salicylates.
Numerous conditions unrelated to thyroid disease can also cause abnormal T3 levels. Accordingly, total T3 levels should not be used as the sole characteristic to determine an individual's thyroid function. Serum levels of T4, TSH and other clinical findings must also be considered.