The two thyroid hormones T3 (also triiodothyronine) and L4 (also L-thyroxine or levothyroxine) are produced in the epithelial cells of the thyroid gland. Their control is subject to the regulatory hormone TSH basal (thyroid-stimulating hormone or thyrotropin), which is produced in the pituitary gland. The classic thyroid diseases related to hormones are hyperthyroidism, hypothyroidism and autoimmune diseases.
What are thyroid hormones?
In terms of hormones that affect thyroid function, it is important to distinguish T3 and T4, which are produced in the thyroid gland itself, and TSH, which is produced in the pituitary gland. The thyroid hormone T3 is also known as triiodothyronine. Some of it is formed directly in the thyroid gland, while others are continuously made available to the body via the conversion of the thyroid hormone T4 into T3. A distinction is made in the blood between the bound form, known as total T3, and the free form. The fT3 occurs in a smaller proportion, but is particularly relevant for meaningful blood tests. The thyroid hormone T4 also exists in the free form, which is then referred to as fT4. T4 is the same as L-thyroxine or levothyroxine. Central regulation of thyroid hormones is carried out by the pituitary gland, which secretes the control hormone TSH (thyroid-stimulating hormone or thyrotropin). In the C cells of the thyroid gland, the hormone calcitonin is formed, which, due to its function, is not one of the actual thyroid hormones.
Anatomy and structure
The classical thyroid hormones are called T3 and T4 because of their molecular structure: The number 3 in triiodothyronine comes from the fact that the hormone has three iodine atoms in its structure. In the case of L-thyroxine or levothyroxine, there are four iodine atoms, hence the abbreviation T4. The formation of these two classic thyroid hormones takes place in the so-called thyrocytes, the follicular epithelial cells of the organ, which is located in a butterfly shape at the front of the neck below the larynx. TSH, on the other hand, is secreted by the pituitary gland – a hormonal gland located in the middle cranial fossa. The pituitary gland is connected to the thyroid gland via a complicated regulatory circuit. It is also called the thyrotropic control circuit and regulates the delivery of thyroid hormones in the required concentration through the bloodstream.
Function and tasks
The functions of thyroid hormones are vital, so they must be compensated for life if the organ is underactive or surgically removed. T3 and T4 have a variety of functions that affect a wide range of organ systems. They are significantly involved in numerous metabolic functions and serve to maintain a properly functioning organism. Among other things, they ensure that the body receives the energy necessary for unrestricted performance. This happens, among other things, because the thyroid hormones play their part in enabling the body to grow and its cells to mature unhindered – even in the fetus, by the way. For this reason, an optimal supply of the hormones is particularly important in children and adolescents. The utilization of nutrients from food is also improved with the help of thyroid hormones. The hormones influence body temperature and the cardiovascular system, control mood and concentration and have a significant influence on fertility. In the case of both T3 and T4, only the free portion is effective, which is not bound to transport proteins in the body. Moreover, the biological effectiveness of fT3 (free triiodothyronine) is several times higher than that of free T4. A superordinate role is played by TSH, which regulates the processes centrally after its release from the pituitary gland. Via a sensitive control mechanism, thyroid-stimulating hormone migrates from the pituitary gland to the thyroid gland and triggers the formation of T3 and T4 there. In a different pathway, thyroid hormones can in turn throttle TSH production in the pituitary gland as part of a negative feedback loop so that, optimally, equilibrium is brought about.
Diseases
Typical diseases related to thyroid hormones are hyperthyroidism or hypothyroidism and the autoimmune diseases Hashimoto’s thyroiditis and Graves’ disease.In hyperthyroidism, the thyroid gland works beyond the required level. The organism runs at full speed. Typical signs include sweating, palpitations and racing heart, diarrhea, weight loss with normal food intake, and nervousness that is often without reason. On the basis of a blood test, hyperthyroidism can be recognized by an increased free T3 and T4 or a decreased TSH. The thyroid-specific laboratory values are reversed in hypothyroidism: TSH is above the norm, free T3 and T4 are too low. The physical and psychological symptoms behave accordingly: a patient with hyperthyroidism often gains weight involuntarily, freezes easily, is often tired and may suffer from constipation. Autoimmune diseases include Graves’ disease and Hashimoto’s thyroiditis. In Graves’ disease, the body makes antibodies against its own thyroid tissue. It is therefore often accompanied by hypothyroidism, the underactivity of the thyroid gland. Other possible symptoms include the well-known goiter in the lower neck and an endocrine orbitopathy, which is noticeable due to clearly protruding eyes. In Hashimoto’s thyroiditis, there are two different variants of the disease. Both develop an underactive thyroid (hypothyroidism), although the initial destruction of the thyroid tissue can also initially manifest itself in an overactive thyroid. If the thyroid gland has been removed, for example due to cancer or a disruptive goiter, lifelong substitution with the vital thyroid hormones is necessary.
