The thyroid gland produces two different hormones, thyroxine (T4) and triiodothyronine (T3). The synthesis and release of these hormones is regulated by the hypothalamus and pituitary gland. Their main purpose is to increase the energy metabolism. The thyroid gland produces the hormones T3 and T4 on the one hand and calcitonin on the other. These hormones are discussed separately below.
Synthesis of thyroid hormones
Through an active mechanism, under the influence of thyrotropin from the pituitary gland, the thyroid gland can absorb iodine from the blood into the thyroid cells (thyroocytes). This happens with the help of a sodium–iodide symporter, which absorbs iodide from the blood under an energy-consuming mechanism. Subsequently, the so-called iodization takes place in the thyroid cells (thyroid gland cells).
Here, the iodide in the cells is first oxidized by the thyroid peroxidase and then attached to the amino acid tyrosine by the iodine transferase. Afterwards, two iodinated tyrosine residues condense with each other and thus form the thyroxine (T4). This is then released from the thyroid cells and stored as thyroglobulin in the thyroid follicles.
Release of thyroid hormones
When the thyroid hormones are to be released, a signal is first sent to the thyroid follicles, which then release the thyroglobulin back to the thyroid cells by endocytosis. In the thyroid cells, the thyroglobulin is transported to the basement membrane. There the thyroglobulin is split off from its carrier substance and free thyroxine (T4) and free triiodothyronine (T3) are produced.
These thyroid hormones are released into the blood in a ratio of 10-20:1. Since only T3 is the biologically active thyroid hormone, it is produced in the blood from T4 by mono-deiodination at the phenol ring. This dejodination is controlled by the individual organs and their activation of the deodase. For this reason, not all of the T4 is directly converted into effective T3, but only when an organ needs the hormone action.
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