Transport in blood

Both thyroxine (T4) and triiodothyronine (T3) are 99% bound to the thyroxine-binding globulin (TBG) in the blood. This serves to transport the hormones and prevents an early effect of T3. Only about 0.03% T4 and 0.3% T3 are present in the blood unbound and therefore biologically active.

The half-life of unbound T4 in the blood is approx. 190 hours, the half-life of effective T3 approx. 19 hours.

Inactivation

The inactivation of the biologically active T3 thyroid hormone takes place in the kidney and liver by a renewed deodorization. The iodine released in this process is again made available to the thyroid gland for renewed hormone synthesis.

Regulation of the thyroid gland function

Thyrotropin (TSH) from the pituitary gland regulates iodine uptake and thyroid synthesis in the thyroid gland. Likewise, the release of T3 and T4 from the thyroid into the blood increases under the influence of thyrotropin. T3 and T4 from the blood then in turn exert a negative feedback on the hypothalamus and pituitary gland.

This means that a high concentration of thyroid hormones in the blood leads to an inhibition of TSH release from the pituitary gland and thus a lower production and release of thyroid hormones in the thyroid gland. If the concentration of hormones in the blood drops, this leads to stimulation of the hypothalamus and pituitary gland, so that more thyroid hormones are produced and released. This mechanism enables an exact regulation of the concentration of thyroid hormones in the blood (euthyroid metabolic state).

Effect of thyroid hormones

In general, only triiodothyronine (T3) is biologically active and activates the entire metabolism. In detail, this means that T3 increases the energy metabolism in a dose-dependent manner. This means an increased activation of the ATP-consuming sodium–potassium pump in the cell walls.

This increases the energy metabolism of the entire body. This is also known as a calorigenic effect and only occurs a few hours to days after thyroid hormone administration. Furthermore, T3 has an effect on the carbohydrate metabolism.

Through increased glycogen breakdown in the liver, it lowers the glycogen content and simultaneously increases the liver’s own glucose production. As a result, T3 has a small effect against insulin, thus slightly increasing the sugar content of the blood. It also has a similar effect on fat metabolism.T3 mobilizes fats from the adipose tissue and thus has a lipolytic effect.

Both the effect on carbohydrate and fat metabolism serve to provide energy sources for consumption within the scope of the caloric effect. In addition, physiological thyroid hormone concentrations have an anabolic effect, i.e. they serve to build muscle. Increased thyroid hormone concentrations, on the other hand, have a catabolic effect, i.e. they promote protein breakdown. In addition, thyroid hormones increase the responsiveness to catecholamines (adrenaline, noradrenaline), which in turn also increase the basal metabolic rate, sugar and fat breakdown.