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If you feel that any of our content is inaccurate, out-of-date, or otherwise questionable, please select it and press Ctrl + Enter.
Free triiodothyronine in blood
Medical expert of the article
Last reviewed: 04.07.2025
Reference values (norm) of cT3 ( triiodothyronine) in blood serum are 4-7.4 pmol/l.
CT3 (triiodothyronine ) accounts for 0.3% of its total amount in the blood. The cT3 (triiodothyronine) fraction provides the entire spectrum of metabolic activity. cT3 ( triiodothyronine) is a product of the metabolic conversion of T4 ( thyroxine) outside the thyroid gland. It should be emphasized that the deiodination of T4 with the formation of T3 ( triiodothyronine) occurs more intensively in the anterior pituitary gland than in peripheral tissues. In this regard, determining the level of cT4 in the serum is of great importance in assessing the state of regulation of the secretion of thyroid-stimulating hormone according to the principle of feedback. The content of cT3 ( triiodothyronine) does not depend on the concentration of TSH, therefore its determination is very informative for assessing the thyroid status with a change in the content of TSH.
Determination of the concentration of CT3 ( triiodothyronine) is justified in the primary diagnosis and long-term monitoring of patients with thyrotoxicosis that developed in iodine-deficient areas, in which suppression of the formation of thyroid-stimulating hormone is possible with isolated hyperproduction of only T3 ( triiodothyronine) and an unchanged level of T4 ( T3 - toxicosis). In conditions of mild iodine deficiency, this condition is often observed in diffuse toxic goiter (up to 25% of cases). In the case of hyperthyroidism, the concentration of CT3 increases, and in hypothyroidism, it decreases.
Diseases and conditions in which the concentration of cT3 ( triiodothyronine) in the blood serum changes
Increased concentration |
Decreased concentration |
Thyrotoxicosis, iodine deficiency Condition after treatment with radioactive iodine preparations Pendred syndrome Use of estrogens, oral contraceptives, methadone, heroin |
Postoperative conditions and severe illnesses Hypothyroidism Acute and subacute thyroiditis Taking androgens, dexamethasone, propranolol, salicylates, coumarin derivatives |
[