Common triiodothyronine (T3)

Common triiodothyronine (T3) Reflects the total concentration of triiodothyronine bound to free and plasma proteins.

Triiodothyronine (T3) Is a thyroid hormone, an iodinated derivative of the amino acid tyrosine.

Most of the T3 hormone is produced extrathyroidically, especially in the liver, as a result of the loss of 4 atom of iodine by T1 (T3 contains 3 atoms of iodine).

About 4% of thyroxine (T30) is metabolized to T3 and to "reverse" (inactive) T3 forms. The latter has no hormonal action and does not inhibit TSH secretion.

About 3% of triiodothyronine (T20) is produced by the thyroid gland under the influence of TSH, and 80% - as a result of deiodination of T4 in peripheral tissues (mainly in the liver and kidneys). Therefore, serum T3 concentration reflects the functional state of peripheral tissues rather than the secretory activity of the thyroid gland.

The hormonal activity of T3 is three times higher than that of T4. Free T3 accounts for 0.3% of total serum hormone levels. More than 3% of T80 is associated with thyroxine-binding globulin (TBG), an increase in the concentration of which causes an increase in serum total T3 and total T4 levels, initially leading to a short-term decrease in free T3 and free T4. The secretions of T3 and T4 then intensify and their total content increases until normal levels of free T3 and T4 are restored.

Thyroid hormones inhibit the secretion of TSH by the principle of negative feedback, affecting the thyrotropic cells of the adenohypophysis.

Triiodothyronine (T3) increases heat production and oxygen consumption by all tissues in the body except the brain, spleen, and testicles. Stimulates the synthesis of vitamin A in the liver. Reduces the concentration of cholesterol and triglycerides in the blood, accelerates protein metabolism. Increases urinary calcium excretion, activates bone metabolism, but more - bone resorption; Has a positive chrono- and inotropic effect on the heart. Stimulates reticular formation and cortical processes in the central nervous system.

The maximum level of T3 falls in the period from September to February, the minimum - in the summer.

Total T3 levels are influenced by two main factors: thyroid secretory intensity and the ability of serum proteins to bind.

When to take a test for total triiodothyronine?

• Assess the functional state of the thyroid gland;
• Prognosis of Bazedov disease;
• Differential diagnostics of thyroid disease;
• Diagnosis of T3 thyrotoxicosis (TSH suppression with normal T4) or cases where FT4 is normal in the presence of signs of hyperthyroidism;
• Monitoring of antithyroid therapy as well as replacement and suppression therapy with high-dose thyroxine (T4) -drugs. Assessment of thyrotoxicosis induced by amiodarone background; Assessment of artificial thyrotoxicosis (against the background of cytomegalovirus);

How to prepare for the test?

The test should preferably be performed on an empty stomach in the morning (around 10 am).

Research material

Venous blood

Possible interpretation of the results

Total triiodothyronine concentration increases:

• Thyrotropinoma;
• Toxic goiter;
• Isolated T3 toxicosis;
• Thyroiditis;
• Thyrotoxic adenoma of the thyroid gland;
• T4-resistant hypothyroidism;
• Thyroid hormone resistance syndrome;
• TSH-independent thyrotoxicosis;
• Postpartum thyroid dysfunction;
• Chorionic carcinoma;
• IgG myelomas;
• Nephrotic syndrome;
• Chronic liver disease;
• Obesity;
• Systemic connective tissue diseases;
• Hemodialysis

Total triiodothyronine concentration decreases:

• Primary, secondary, tertiary hypothyroidism;
• Primary decompensated adrenal insufficiency;
• Severe stress, hunger (low-fat diet), smoking;
• Chronic liver disease (e.g. cirrhosis);
• Severe thyroid pathology, including somatic and mental illness; Recovery period after serious illness;
• Artifact of thyrotoxicosis due to arbitrary administration of thyroxine-containing preparations;
• Euthyroid syndrome.

Resources

https://spravochnik.synevo.ua/ru/thyrioidnaia/triiodthyronine-obshiy.html