Tasks/function of thyroxine
Hormones are the so-called “messenger substances of the body”. They are transported with the blood and pass on their information to the cells at their destination via a variety of different routes. Thyroid hormones even transmit their signals directly to the DNA.
They bind directly to it and promote the reading of the corresponding information, which is decisive for its effect. The disadvantage is that it takes much longer to implement an effect via the DNA. The advantage, however, is that both the lifespan of the hormones and the effect are longer.
The two thyroid hormones, thyroxine and triiodothyronine, differ only in their potency and can be converted into each other. Therefore, when we talk about thyroxine in the following, we also mean triiodothyronine. The most important tasks of the thyroid gland are energy metabolism and growth.
Thyroxine promotes energy metabolism by increasing the amount of free sugar in the blood, which acts as an energy supplier. To do this, the body’s own production of sugar molecules is increased on the one hand, and on the other hand the existing sugar storage is broken down and released into the blood. In addition to the supply of sugar, another important supplier is provided, namely fats.
Thyroxine promotes the breakdown of stored fat, which is also converted into energy in a more complex process. Another important effect is the reduction of plasma cholesterol by promoting the cholesterol metabolism of the cells. The conversion of sugar and fat into energy also generates heat.
This is further enhanced by a further, more complicated effect of thyroxine, which is why, for example, patients with hyperthyroidism often sweat and wear only light clothing even on colder days. In addition to energy metabolism, the second major effect of thyroid hormones is seen in growth.This plays an important role especially in children and adolescents and is therefore examined in the newborn screening. Thyroxine promotes the growth and maturation of cells, especially through the release of further growth hormones, and is also important for brain development in newborns.
If hypothyroidism is not detected and treated in time, growth and developmental disorders can therefore occur. In addition to its two main functions, thyroxine also acts on the connective tissue and has a promoting function there. Patients with hypothyroidism may therefore develop so-called “myxedema”.
The heart is also affected by thyroxine. There, it causes both an increase in the heart rate and an increase in the strength of contraction. As already mentioned, the thyroid gland produces triiodothyronine (T3) in addition to thyroxine (T4).
Although the two hormones have the same effect, they differ in their potency. T3 has an effect about three times as strong as T4. Therefore, a large proportion of T4 (about 30%) is subsequently converted into T3. However, triiodothyronine is not very stable and only survives in the blood for about one day.
