Free triiodothyronine (FT3)

Triiodothyronine (T3) Is a thyroid hormone, most of which circulates in a state bound to proteins - thyroxine-binding globulin and albumin.

About 3% of circulating triiodothyronine (T80) is produced by peripheral transformation of thyroxine (T4), while the remaining 20% ​​is produced by the thyroid gland itself.

It represents only 0,3% of the total amount of triiodothyronine in the blood Free triiodothyronine (FT3), which provides the entire range of metabolic activity. FT3 is almost 5 times more active than FT4. It increases heat production and oxygen consumption in all body tissues except brain tissue, spleen and testes; stimulates the synthesis of vitamin A in the liver; reduces the concentration of cholesterol and triglycerides in the blood; accelerates protein metabolism; increases the excretion of calcium in the urine; activates bone metabolism, but more - bone resorption; It has a positive chrono- and inotropic effect on the heart; Stimulates reticular formation and cortical processes in the central nervous system. The concentration of T3 depends on the concentration of TSH (thyreotropic hormone), so its determination is informative to assess the status of the thyroid gland when the level of TSH changes.

In thyrotoxicosis, as a rule, the concentration of T4 and T3 (total and free) in the blood is increased, but in a small number of patients with T3-toxicosis, only the level of T3 is increased. This is usually associated with iodine deficiency, when TSH levels can be suppressed by isolated hyperproduction of T3 with unchanged T4 concentrations.

Serum T3 and FT3 concentrations are substantially dependent on the T4 → T3 peripheral conversion rate. The conversion speed can be reduced:

• In the case of serious systemic diseases caused by non-thyroid causes (progressive neoplasia, decompensated cirrhosis of the liver, end-stage renal failure, sepsis, anorexia nervosa), which can lead to "T3 depression syndrome", characterized by a decrease in T3 / FT3.
• After the appointment of certain drugs: corticosteroids, propranolol and amiodarone;
• In old age: T3 / FT3 concentration is 10-50% lower than in young people (for this reason, a mild form of hyperthyroidism can be ignored in this age group).

When should we test for free triiodothyronine?

• Detection of isolated secretion of triiodothyronine (T3 hyperthyroidism), which may occur in approximately 10% of cases of hyperthyroidism;
• Identify patients with subclinical hyperthyroidism who present with TSH inhibition and normal concentrations of FT4 and FT3; These individuals have a high risk of developing overt clinical hyperthyroidism;
• Prognosis of Graves' disease;
• Detection of relapse in patients with hyperthyroidism (increased FT3 levels may be an early sign in this case);
• Assessment of the severity of primary hypothyroidism;
• Monitor levothyroxine treatment (to prevent overdose).

How to prepare for the test?

The test should be performed on an empty stomach.

Research material

Venous blood

Possible interpretation of the results

Concentration of FT3 increases:

• Thyrotropinoma;
• Diffuse toxic goiter;
• Isolated T3-toxicosis;
• Thyroiditis;
• Thyrotoxic adenoma;
• T4-resistant hypothyroidism;
• Thyroid hormone resistance syndrome;
• TSH-independent thyrotoxicosis;
• Postpartum dysfunction of the thyroid gland;
• Chorionic carcinoma;
• Decreased levels of thyroxine-binding globulin;
• Myeloma with high levels of IgG;
• Nephrotic syndrome;
• Hemodialysis;
• Chronic liver disease.

FT3 concentration decreases:

• Uncompensated, primary insufficiency of the adrenal gland;
• Severe non-thyroid pathology, including somatic and mental illness; Recovery period after a serious illness;
• Primary, secondary, tertiary hypothyroidism;
• Low-calorie and low-calorie diet;
• Heavy physical activity in women;
• Weight loss.

Resources

https://spravochnik.synevo.ua/ru/thyrioidnaia/triiodthyronine-svobodniy.html
https://www.synevo.ro/shop/ft3-triiodotironina-libera/