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If you feel that any of our content is inaccurate, out-of-date, or otherwise questionable, please select it and press Ctrl + Enter.
Total triiodothyronine in blood
Medical expert of the article
Last reviewed: 05.07.2025
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.