Free Thyroxine (FT4)

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Additional information

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Free Thyroxine (FT4) Is a biologically active fraction of thyroxine that circulates in the blood, is not related to blood proteins and makes up 4% of total thyroxine (T0.03).

Free thyroxine (FT4) is produced by the follicular cells of the thyroid gland under the influence of TSH and is regulated by the so-called negative feedback principle.

Thyroxine (T4) Is Triiodothyronine (T3) Predecessor. It increases heat production and oxygen consumption by all tissues of 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.

Under normal thyroid function, the amount of free T4 in the blood does not depend on the concentration of thyroxine-binding globulin (TBG), therefore Free thyroxine (FT4) most adequately and directly reflects thyroid hormonal function.

A combined study of free thyroxine (FT4) and thyroid-stimulating hormone (TSH) gives a more complete picture of the thyroid gland in patients who have altered thyroxine-binding globulin (TBG) levels (pregnancy; intake of androgens, estrogens, etc.).

During the day, the maximum concentration of thyroxine is set from 08:00 to 12:00, the minimum - from 23:00 to 03:00. During the year, the maximum levels of thyroxine are observed from September to February, and the minimum - in summer. In women, thyroxine concentrations are lower than in men.

When should we test for free thyroxine?

Often, Free thyroxine (FT4) is tested in combination with thyroid stimulating hormone (TSH), When we suspect thyroid damage. Thyroid hormone levels may be elevated or decreased. In addition, to test the full functional state of the thyroid gland, it is directly recommended to test the following combination of hormones: FT3, FT4 and TSH.

Determination of free thyroxine (FT4) is important in the complex assessment of thyroid function.

The study of free and metabolically active thyroxine is recommended in patients with symptoms of elevated or impaired thyroid function. The test is also useful in people with an enlarged gland (so-called "goiter") or "nodules", especially when TSH levels are not within the normal range.

The determination of FT4 is also informative for monitoring thyroid-suppressing therapy - replacement therapy with thyrostatics and thyroid hormones.

How to prepare for the test?

The test is taken on an empty stomach, in the morning (until about 10 o'clock). Drinking coffee and smoking before testing will seriously affect the results. It is allowed to drink 250 ml of water before testing.

Do not take multivitamins or dietary supplements containing biotin (vitamin B12) for 7 hours before testing.

In hypothyroid patients, after the appointment of thyroxine, the level of free thyroxine increases and reaches a maximum in 1-6 hours after taking the drug; Therefore, the sample should be taken immediately before the next dose of the drug.

If a patient is being treated with thyroxine-containing agents, blood collection for FT4 should be performed 4-6 weeks after cessation of treatment.

Research material

Venous blood

Possible interpretation of the results

Elevated rates indicate the presence of hyperthyroidism or exogenous thyroxine, decreased rates - hypothyroidism.

Free thyroxine (FT4) reflects the total amount of thyroxine (T4) well, even under altered amounts of thyroxine-binding globulin (TBG). Sometimes, hormone levels can be misleading if there are other abnormal proteins in the patient's blood that can retain the hormone, or if the patient has other serious illnesses.

FT4 level increased:

  • Hyperthyroidism;
  • Acute thyroiditis;
  • Pregnancy;
  • Obesity;
  • Hepatitis

FT4 level is low:

  • Hypothyroidism;
  • Increased protein loss (renal syndrome, diseases of the gastrointestinal tract);
  • Itzenko-Cushing syndrome;
  • Significant iodine deficiency;
  • Physical activity;
  • Panhypopituitarism;
  • Increased loss of protein from the digestive tract.


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