Formation of T3T4: These thyroid hormones are formed in the thyroid gland, or more precisely in its follicles (roundish structures of cells), from the amino acid thyrosine. There are two types of hormones known as thyroid hormones. T4 hormones occur in the blood about 40 times more than T3 hormones, but T3 acts faster and about three to eight times stronger than T4 and can be produced from T4 with the help of the enzyme deiodase.
This is done in the liver, kidneys or the target cells of the hormones by splitting off iodine from T4. In this way, 80% of the amount of T3 is produced; only the remaining 20% of the hormone is produced directly in the thyroid gland. T4 thus acts as a kind of “hormone store” in the blood and T3 is the effective hormone.
- Thyroxine or tetraiodothyronine (T4) and
- Triiodothyronine (T3)
First, these hormones are stored in the follicles of the thyroid gland bound to the protein thyroglobulin. After their release into the blood, the thyroid hormones are found coupled to a total of three transport proteins: In addition, there are free thyroid hormones in plasma which are not bound to proteins. However, these only make up a proportion of less than 0.3%.
The receptor to which the thyroid hormones bind is located intracellularly.
- Thyroxine-binding globulin (TBG)
- Thyroxine binding prealbumin (TBPA)
- Serum Albumin
Regulation of T3 and T4 hormones: The hormonal control circuit of these hormones consists of the thyroliberin (TRH)-thyrotropin (TSH) axis, which regulates the release of T3 and T4 hormones. TRH is produced as liberin in the hypothalamus and unfolds its effect on the pituitary gland, which releases TSH through TRH stimulation.
This in turn acts on the thyroid gland so that thyroid hormones are secreted. TSH secretion is prevented by the statin somatostatin (SIH), which is also produced in the hypothalamus. Negative feedback from the thyroid hormones themselves regulates their release by inhibiting TRH secretion from the hypothalamus and reducing the amount of TRH receptors on the pituitary gland.
As a result, if the amount of thyroid hormones is large, less TRH is released or its effect on the pituitary gland is reduced by fewer “docking possibilities” (receptors), so that the concentration of thyroid hormones no longer increases and the amount decreases overall due to the reduction of the already existing hormones. Generally speaking, the hormones of the thyroid gland are responsible for growth, maturation and metabolism. More precisely, these hormones increase the body’s energy metabolism, resulting in increased heat production. They are also responsible for growth and maturation, especially of the brain and bones, and have an influence on other hormones, namely insulin, glucagon, STH and adrenaline.
