Introduction
Thyroxine, or “T4”, is a hormone produced in the thyroid gland. Thyroid hormones have a very broad spectrum of activity and are of great importance, especially for energy metabolism, growth and maturation. Since thyroid hormones, and thus also thyroxine, are subject to a superordinate and very complex control circuit and depend on the presence of “iodine“, the thyroid gland is very susceptible to functional disorders. Over- and underfunctions of the thyroid gland are therefore a very common clinical picture.
Structure of thyroxine
Thyroxine is produced and released in the thyroid gland. Among other things, it consists of two “molecular rings”, which are connected to each other via an oxygen atom. There are a total of four iodine atoms on the two rings, two each on the inner and outer ring.
For this reason, thyroxine is also called “T4” or “tetraiodothyronine”. The iodine thus represents an important building block in the synthesis of thyroid hormones. It is absorbed from the blood into the thyroid gland and immediately converted so that it cannot leave it again.
This mechanism is also known as the “iodine trap”. Since iodine is so essential for the synthesis of thyroid hormones and thus for their function, there should always be a sufficient supply of iodine in the body, otherwise there is a risk of thyroid hypothyroidism. This was a common problem, especially in earlier times, because iodized salt was not yet available.
Nowadays, iodine deficiency is a rather rare cause of hypothyroidism in Europe. The exact structure of thyroxine is very important for its function, since even a small difference can cause a large change in its effect. The second important thyroid hormone “T3” or “triiodothyronine” serves as a good example.
It differs from T4 only in that it has one iodine less on the outer ring and therefore only three iodine atoms in total. Thyroid hormones are fat-soluble molecules. This means that they only dissolve in fatty substances and “precipitate” in water.
This is roughly the same as when someone drops a drop of fat in water and hopes that it will dissolve. Since thyroxine, like all hormones, is transported in the body with the blood and this is very watery, it has to be bound to a transport protein. Bound to the protein, thyroxine survives in the body for about a week. When the hormone has reached its destination, it separates from the transport protein and crosses the cell membrane of the target cell, where it unfolds its effect.
