Triiodothyronine (T3) is the main thyroid hormone biologically active. T3 mainly circulates bound to transportation proteins, such as albumin, prealbumin and TBG (Thyroid Binding Globulin); only the free fraction
Triiodothyronine (T3) is the main thyroid hormone biologically active. T3 mainly circulates bound to transportation proteins, such as albumin, prealbumin and TBG (Thyroid Binding Globulin); only the free fraction (fT3), which represents about 0.3% of the circulating T3, is considered responsible for the biological activity. The total T3 concentration not only depends on the thyroid conditions of the subject and on the peripheral conversion from T4, but also on the carrier proteins concentration, varying with the variation of their blood levels.
The fT3 concentration, instead, does not depend on such factors and therefore it is able to show the thyroid conditions of the patient, with more reliable results from a diagnostic point of view.
The fT3 assay is important in case of hyperthyroidism (Graves’ disease or toxic adenoma), in case of monitoring of hypothyroid patients treated with Thyroxine (T4) and anti-thyroid medications and in case of low T3 syndrome (Hashimoto’s thyroiditis). For a correct diagnosis, the fT3 assay must be associated to the fT4 and TSH assay, and to the patient’s clinical data.
