Pharmacokinetics and pharmacodynamics (mode of action)
As already described, the diet contains almost exclusively iodine in the form of its salts, i.e. in the form of iodide. In the gastrointestinal tract, this is absorbed and passes into the so-called extracellular fluid, i.e. the fluid that is present between the cells. Iodine, which is released by the iodine-containing thyroid hormones during decomposition, is also found in this space.
Thus the extracellular fluid serves as an iodide pool. From the extracellular fluid, the iodide is then transported via a transport mechanism to a specific cell type of the thyroid gland, the follicular epithelium cell. In the cells, the iodide diffuses into the upper parts of the cell space where it is transported across the cell membrane by transport proteins.
During this process, an enzyme called thyroperoxidase (TPO) initiates a chemical reaction that ensures that the iodide finally turns into activated iodine. After this reaction, the activated iodine can be incorporated into certain amino acid residues (tyrosine residues) of thyroglobulin (TG). Thyroglobulin is a protein of the thyroid gland where the two thyroid hormones are produced.
The incorporation of one iodine atom produces the intermediate product monoiodotyrosine (mono = one), the incorporation of another iodine atom produces diiodtyrosine (di = two). Now the enzyme thyroperoxidase (TPO) is used again. This enzyme now ensures that a diiodtyrosine is linked with another diiodtyrosine.This produces thyroxine (tetraiodothyronine, T4).
If, on the other hand, the enzyme combines monoiodtyrosine with diiodtyrosine, triiodothyronine (T3) is formed. The two products thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3) are the actual thyroid hormones which are stored in the thyroid follicle bound to the protein thyroglobulin (TG). Thyroid follicles are closed compartments inside the thyroid gland. When thyroid hormones are needed in the organism, they are first absorbed into the thyroid cells, where the thyroglobulin is consequently dissolved and the thyroid hormones bound to it are released. The thyroid hormones are finally released from the thyroid cells into the organism by the stimulus of the thyroid-stimulating hormone (TSH), which originates from the front lobe of the pituitary gland (adenohypophysis), and can thus unfold their effect.
