Free thyroxine in the blood
Last reviewed: 23.04.2024
All iLive content is medically reviewed or fact checked to ensure as much factual accuracy as possible.
We have strict sourcing guidelines and only link to reputable media sites, academic research institutions and, whenever possible, medically peer reviewed studies. Note that the numbers in parentheses ([1], [2], etc.) are clickable links to these studies.
If you feel that any of our content is inaccurate, out-of-date, or otherwise questionable, please select it and press Ctrl + Enter.
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.
[