Thyroid gland hormone, which is responsible for the normal functioning of the entire body.

The main function of T3 is to regulate basal metabolism due to increased oxygen consumption by cells. The basic metabolism is the minimum amount of calories that a person needs for normal functioning. T3 accelerates the rate of protein creation, the rate of "burning" glucose and fats (cholesterol).

T3 affects almost every organ except the spleen and testicles. Under the influence of thyroxine, the heart rate increases and blood pressure increases. The body temperature rises. The hormone is responsible for the growth and development of the body – the central nervous system and bone structure. In addition to the thyroid gland, the hormone can be formed in the peripheral blood due to the cleavage of the iodine molecule from the hormone T4. T3 has an effect 3-5 times stronger than T4, however, the lifetime of T3 is from 1-2.5 days, and T4 is 5-7 days, so the concentration of T3 in the blood is almost 60 times less than T4.

The amount of T3 is directly related to the level of iodine and TSH (see thyroid-stimulating hormone). Excess hormone is called "hyperthyroidism" and manifests itself as follows:
- Accelerated heartbeat (tachycardia), increased blood pressure, possible rhythm disturbance;
- Increased body temperature, sweating;
- Nervousness, fatigue, weakness with increased physical activity, hand tremors;
- Sleep disturbance;
- Decrease in body weight, despite increased appetite, diarrhea occurs.

Deficiency ("hypothyroidism") has the following clinical manifestations:
- Drowsiness, lethargy, memory loss;
- Weight gain, edema;
- Dry skin;
- Tendency to constipation;
- Reproductive dysfunction in women.

Total T3 includes two fractions: bound and free. The functional fraction is free T3, the concentration of which is about 0.3% of the total amount of the hormone. Various factors (for example, the level of thyroxine-binding protein) lead to the fact that with a normal total amount of hormone, the content of free T3 changes, therefore, the activity of the hormone also changes. For this reason, it is recommended that total and free triiodothyronine be determined simultaneously to adequately portray a picture of T3 hormone activity. The test is particularly informative in the initial diagnosis and monitoring of the effectiveness of thyrotoxicosis treatment.