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Total triiodothyronine in blood
Last reviewed: 05.07.2025
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Reference values (norm) for T3 (triiodothyronine) concentration in blood serum are 1.08–3.14 nmol/l.
T3 (triiodothyronine ) is formed and synthesized by the thyroid gland, but the majority of T3 is formed outside the thyroid gland by deiodination of T4. Approximately 99.5% of T3 circulating in the blood is bound to proteins. The half-life in the blood is 24-36 hours. The activity of T3 is 3-5 times greater than the activity of T4.
Determination of triiodothyronine is very informative in T3 - thyrotoxicosis , since in a number of cases the concentration of T4 does not change significantly (only the concentration ofT3 is sharply increased ).
In myeloma, which produces a large amount of IgG, as well as in severe liver diseases, falsely elevated values of triiodothyronine concentration are recorded.
In elderly people, as well as in patients with severe somatic diseases, the so-called low T3 syndrome is often observed - a decrease in its concentration in the blood serum with a normalT4 content. Low triiodothyronine syndrome in this group of people is not a sign of hypothyroidism.