Thyroxine (T4) is the main hormone produced by the thyroid, stimulated by the human thyroid-stimulating hormone (hTSH). T4 mainly circulates bound to transportation proteins, such as albumin, prealbumin and TBG
Thyroxine (T4) is the main hormone produced by the thyroid, stimulated by the human thyroid-stimulating hormone (hTSH). T4 mainly circulates bound to transportation proteins, such as albumin, prealbumin and TBG (Thyroid Binding Globulin); only the free fraction (fT4), which represents about 0.03% of the circulating T4, is indeed active. The total T4 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 fT4 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 fT4 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 T4 syndrome (Hashimoto’s thyroiditis). For a correct diagnosis, the fT4 assay must be associated to the fT3 and TSH assay, and to the patient’s clinical data.
