The main thyroid hormone.

T4 affects almost every organ except the spleen and testicles. Thyroxine increases heart rate and blood pressure. It accelerates the rate of protein creation, the rate of glucose and fat (cholesterol) burning, and thickens the inner layer of the uterus (endometrium) in women. The body temperature rises. The hormone is responsible for growth and development of the body - the central nervous system and bone structure. The biological activity is much lower than that of T3, but the elimination time and concentration are much higher.

T4 has a peculiarity: under the influence of certain enzymes, it is converted into a more active form of the hormone, T3.

The amount of T4 is directly related to iodine levels and TTH (see thyroid hormone).

This hormone affects our behaviour: its increase leads to higher activity, short temper, violent mood swings and a tendency to have hysterical seizures. T4 deficiency in the body manifests itself as lethargy, tendency to tearfulness and depression, lethargy.

Total T4 includes two fractions: bound and free. The functional fraction is free T4, whose concentration is about 0.03% of the total hormone. According to some sources (Dr. A.A. Kishkun, "Guidelines for Laboratory Diagnostic Methods", 2007) free T4 has one important advantage over total T3 (see "free T3 (triiodothyronine)). Its concentration is independent of thyroxine-binding globulin (TSH), a protein which makes thyroid hormones 'pro-free'. Consequently, this feature makes free thyroxine the most accurate indicator of thyroid hormone function, regardless of the amount of TSH distorting total T4 but not affecting free T4.