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Free thyroxine in the blood

Alexey Kryvenko, medical expert
Last reviewed: 05.07.2025

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Reference values for cT4 ₍ thyroxine) in the blood are 10-35 nmol/l.

CT4 (thyroxine ₎ accounts for 0.03% of its total amount in the blood. During normal thyroid function, the mechanisms that regulate its function work in such a way that the content of CT4 ₍ thyroxine) does not depend on the concentration of TSH. It is this circumstance that allows using CT4 ₍ thyroxine) as the most adequate and direct marker of the hormonal function of the thyroid gland.

In hyperthyroidism, the concentration of cT4 ₍ thyroxine) in the blood increases, and in hypothyroidism, it decreases. An increase in the level of cT4 ₍ thyroxine) is recorded in patients receiving replacement therapy with sodium levothyroxine. Determination of cT4 ₍ 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 an anomaly in the structure of the TSH molecule).

Independence of the concentration of cT4 ₍ thyroxine) from the TSH content allows it to be used as a reliable diagnostic parameter in all conditions accompanied by a change in the concentration of TSH. In this regard, the analysis of cT4 ₍ thyroxine) is indispensable during pregnancy, in women taking oral contraceptives or receiving estrogens or androgens, as well as in individuals with a hereditary increase or decrease in the concentration of TSH. Medicines (salicylates, phenytoin), which distort the results of T4 (thyroxine) determination _, do not affect the true content of cT4 ₍ thyroxine). In some cases, the cT4 test _must be supplemented with other markers: T3 _, cT3 _, TSH.

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