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Free thyroxine in the blood
Last reviewed: 23.04.2024
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The reference values of cT 4 (thyroxine) in the blood are 10-35 nmol / l.
On cT 4 (thyroxine) is 0.03% of its total amount in the blood. With the normal functioning of the thyroid gland, the mechanisms that regulate its function work in such a way that the content of cT 4 (thyroxine) does not depend on the concentration of TSH. This circumstance allows the use of PT 4 (thyroxine) as the most appropriate and direct marker of thyroid hormone function.
In hyperthyroidism concentration Ct 4 (thyroxine) in blood is increased and decreased in hypothyroidism. Improved CT 4 (thyroxine) are detected in patients who receive substitution therapy levothyroxine sodium. The determination of cT 4 (thyroxine) has advantages for the diagnosis of secondary / tertiary hypothyroidism associated with pathology at the hypothalamic-pituitary level, when the concentration of TSH, contrary to the expected decrease, may be within the normal range or even paradoxically increase (due to the abnormality of the structure of the TSH molecule).
Independence concentration Ct 4 (thyroxine) of TBG content allows its use as a reliable diagnostic parameter under all conditions involving a change in the concentration of TBG. In connection with this analysis of the PT 4 (thyroxine) is indispensable for pregnancy, in women taking oral contraceptives or receiving estrogens or androgens, as well as individuals with hereditary increased or decreased concentration of TSH. Drugs (salicylates, phenytoin), which distort the results of determination of T 4 (thyroxine), do not affect the actual content Ct 4 (thyroxine). In some cases, a PT test 4 must be supplemented with other markers: T 3, Ct 3, TSH.
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