Total Thyroxine (T4)

Total Thyroxine (T4 Total) Reflects in the blood Total concentration of free and bound to plasma proteins thyroxine.

Thyroxine (T4) - The amino acid tyrosine is iodinated, contains 4 iodine atoms, is secreted by the thyroid gland under the control of TSH; Plays an important role in general metabolism. 0.05% of the hormone is in the free state, the rest of T4 is bound to thyroxine-binding globulin (TBG), prealbumin, and albumin.

Since the concentration of transport proteins in serum depends on exogenous and endogenous factors (e.g., it increases during pregnancy, decreases after taking oral contraceptives, decreases in nephrotic syndrome), this factor should be taken into account when assessing T4 levels. An increase in the concentration of thyroxine-binding globulin leads to an increase in the level of total T3 and total T4 in the blood serum, which initially leads 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 TSH secretion by the principle of negative feedback, affecting the thyrotropic cells of the adenohypophysis.

Peripheral tissues and the pituitary gland contain a deiodinase complex that converts thyroxine (T4) to triiodothyronine (T3), which in turn is the major active form of the hormone and acts on cells.

During the day, the maximum concentration of thyroxine is set from 8 to 12 hours, the minimum - from 23 to 3 hours. During the year, the maximum values ​​of T4 are observed from September to February, and the minimum - in summer. During pregnancy, total thyroxine concentration increases and reaches maximum values ​​in the III trimester.

When should we take a test for total thyroxine?

Marker of thyroid function:

• Diagnosis of hyperthyroidism;
• Diagnosis of hypothyroidism;
• Monitoring of TSH inhibited therapy.

How to prepare for the test?

Required for analysis Venous blood.

The test should be taken on 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.

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

Possible interpretation of the results

Total thyroxine (T4 Total) levels increase:

• Hyperthyroidism;
• Acute thyroiditis;
• Pregnancy;
• Thyroxine treatment;
• Obesity;
• Hepatitis.

Total thyroxine (T4 Total) levels decrease:

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