The thyroid is a hormone secretory organ. It works according to the so-called thyrotropic control circuit, which is stretched between the thyroid gland and the pituitary gland. This regulatory mechanism controls the concentration of thyroid hormones in the plasma. One of the most important thyroid hormones is thyroxine, which acts as a prohormone on receptors in the central nervous system.
The clinical term euthyroidism refers to the normal state of a healthy thyroid gland and thus an undisturbed closure of the thyroidotropic control loop with a fully functioning thyroid gland. Mostly the term does not correspond to the expression 'healthy thyroid', but rather is used in connection with goitering. In an euthyroid goiter, the thyroid is enlarged, but it works normally.
In the case of thyroid hormone treatment of hypothyroidism or similar diseases, euthyroidism is also often referred to as soon as treatment yields normal thyroid levels. In the case of phenomena such as functional autonomy of the thyroid gland, euthanasia is rarely mentioned, as the activities associated with it usually produce a hormone balance that meets the needs.
The thyroid is a hormone gland and produces the hormones triiodothyronine, thyroxine and calcitonin. Evolutionary, it comes from the epithelial tissue of the second gill arch. It consists anatomically of two lobes, which connects a narrow bridge. The thyroid lobes are about the size of an olive.
The task of the organ is mainly the production of iodine-containing thyroid hormones, which are present in almost all cells of the body and stimulate the energy metabolism. The production of these hormones is regulated by the hypothalamic hormone TRH. This hormone, also known as TSH releasing hormone, regulates thyroid activity in conjunction with the pituitary's thyroid-stimulating hormone.
This thyrootropic control loop has a precisely matched physiology. The pituitary secretes the control hormone thyrotropin to stimulate the thyroid gland to secretion thyroxine and triiodothyronine. This process is fed back. The thyroid hormones inhibit the secretion of TSH by a negative feedback, in order to keep the level of all hormones involved in balance. In turn, the secretion of TSH is dependent on the level of the releasing hormone from the hypothalamus. This hormone of the hypothalamus sets the target value for the thyrotropic control loop.
In addition to this loop, additional feedback loops are involved in the regulation of thyroid pituitary activity. One of them is the ultrashort feedback mechanism of the TSH. In this case, the distribution of TSH in the context of a Brokken-Wiersinga-Prummel control loop has its own secretion. In addition to this principle, the long-feedback mechanism of thyroid hormones also plays a role in TRH secretion and, ultimately, thyroid secretion. The same applies to the control circuits of the plasma protein binding of triiodothyronine and thyroxine.
The thyrootropic control loop can be in different states. When a normal condition is present and the loop is closed when the thyroid is functioning, the doctor speaks of euthyroidism. Deviations from the euthyroid norm state of the control loop arise, for example, in the context of a thyrotoxic crisis, hypothyroidism and thyroid hormone resistance.
The term euthyroidism rules out a malfunction of the thyroid gland. Diseases of the thyroid are not necessarily excluded with the term. The term merely indicates the exclusion of symptoms that are noticeable in the thyrotropic control loop.
The thyrootropic control circuit can even out of balance due to various diseases. Hypothyroidism is a possible cause. The thyroid gland secretes in the context of this phenomenon only a small amount of thyroid hormones. The cause of this phenomenon may be in the thyroid itself or in organs acting on it, such as the hypothalamus.
Primary hypothyroidism also destroys euthyroidism of the thyroidotropic loop. Primary hypothyroidism occurs when the loop in the thyroid has been broken. This may be the case, for example, in the context of a lack of inkretion, as it may occur postoperatively. Another cause of the phenomenon described are autoimmune thyroid disorders, in which the immune system is directed against the structures involved.
Secondary hypothyroidism may also move the thyroidotropic loop out of its normal state. In this phenomenon, the loop is interrupted not in the thyroid, but in the pituitary gland, as it may be in the context of HVL insufficiency. In tertiary hypothyroidism, euthyroidism is disturbed by a missing setpoint due to TSH deficiency. This condition mainly occurs with a lesion of the hypothalamus. All hypothyroidism is based on a specific deficiency.
This should be distinguished from the pathological state of hyperthyroidism, which is synonymous with hyperthyroidism and also disturbs the euthyroidism. Primary hyperthyroidism is the result of pathological over-secretion of thyroid hormones as a result of thyroid disease. The causative disease may, for example, correspond to autonomies or Graves' disease. Secondary hyperthyroidism, on the other hand, is caused by tumors associated with TSH-producing pituitary tumors.
The thyroidotropic loop may also give way to thyrotoxicosis. In such a case, there is an oversupply of thyroid hormones, as it can occur in the context of hyperthyroidism or various medication. A special case of thyrootropic imbalance is thyroid hormone resistance, in which the pituitary-thyroid regulatory circuit at the pituitary receptors is disrupted.Tags: