The term euthyroidism refers to the normal state of the pituitary-thyroid regulatory circuit, thus assuming adequate hormonal function of the two organs. The regulatory circuit is also called thyrotropic circuit. In various thyroid, pituitary, and hypothalamic diseases, it moves outside euthyroidism.
What is euthyroidism?
The clinical term euthyroidism refers to the normal state of a healthy thyroid gland and thus an undisturbed closure of the thyrotropic control loop with a fully functional thyroid gland. The thyroid gland is a hormone-secreting organ. It operates according to the so-called thyrotropic regulatory circuit, which is stretched between the thyroid gland and the pituitary gland. This regulatory mechanism controls the concentration of thyroid hormones in 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 thyrotropic control loop with a fully functional thyroid gland. In most cases, the term does not correspond to the expression ‘healthy thyroid gland’, but is rather used in connection with goiter formation. In euthyroid goiter, the thyroid gland is enlarged but functions normally. In thyroid hormone treatment for hypothyroidism or similar conditions, euthyroidism is also often referred to as soon as the treatment yields normal thyroid levels. Phenomena such as functional autonomy of the thyroid gland are rarely referred to as euthereosis because the associated activities usually establish hormone balance as needed.
Function and task
The thyroid gland is an endocrine gland and produces the hormones triiodothyronine, thyroxine, and calcitonin. Developmentally, it originates from the epithelial tissue of the second gill arch. Anatomically, it consists of two lobes joined by a narrow bridge. The thyroid lobes are about the size of an olive. The function of the organ is primarily the production of iodine-containing thyroid hormones, which are present in nearly all cells of the body and stimulate energy metabolism. The production of these hormones is subject to regulation by the hypothalamic hormone TRH. This hormone is also known as TSH-releasing hormone and regulates thyroid activity together with thyroid-stimulating hormone from the pituitary gland. This thyrotropic regulatory circuit has a precisely tuned physiology. The pituitary gland secretes the control hormone thyrotropin to stimulate the thyroid gland to secrete thyroxine and triiodothyronine. This process is feedback-regulated. Thus, thyroid hormones inhibit the secretion of TSH by feedback to keep the levels 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 from the hypothalamus provides the set point for the thyrotropic control loop. In addition to this control loop, other feedback loops are involved in the regulation of thyroid-pituitary activity. One of these is the ultrashort feedback mechanism of TSH. Here, the secretion of TSH feeds back on its own secretion as part of a Brokken-Wiersinga-Prummel feedback loop. In addition to this principle, the long-feedback mechanism of thyroid hormones also plays a role in TRH secretion and thus ultimately in thyroid secretion. The same is true for the plasma protein binding control circuits of triiodothyronine and thyroxine. The thyrotropic control circuit can be in different states. If a normal state is present and the control loop is closed with a functioning thyroid gland, the physician speaks of euthyroidism. Deviations from the euthyroid normal state of the regulatory circuit occur, for example, in the context of a thyrotoxic crisis, hypothyroidism, and thyroid hormone resistance.
Diseases and disorders
The term euthyroidism excludes thyroid dysfunction. However, diseases of the thyroid gland are not necessarily excluded by the term. The term merely indicates the exclusion of symptoms that are apparent in the thyrotropic control circuit. The thyrotropic control circuit can itself become unbalanced due to various diseases. Hypothyroidism is one possible cause.In the context of this phenomenon, the thyroid gland secretes thyroid hormones only to a small extent. The cause of this phenomenon may lie in the thyroid gland itself or equally in organs acting on it, such as the hypothalamus. Primary hypothyroidism also destroys the euthyroidism of the thyrotropic control circuit. Primary hypothyroidism occurs when the control circuit has been interrupted in the thyroid gland. This may be the case, for example, in the context of a deficient incretion, as may occur postoperatively. Another cause of the phenomenon described are autoimmune thyreopathies, in which the immune system is directed against the structures involved. Secondary hypothyroidism also moves the thyrotropic control circuit out of its normal state if necessary. In this phenomenon, the control loop is interrupted not in the thyroid gland but in the pituitary gland, as may be the case in HVL insufficiency. In tertiary hypothyroidism, on the other hand, euthyroidism is disturbed by a lack of set point due to TSH deficiency. This condition presents primarily with a lesion of the hypothalamus. All hypothyroidism is based on a specific deficiency. To be distinguished from this is the pathological state of hyperthyroidism, which is equivalent to hyperthyroidism and also interferes with euthyroidism. Primary hyperthyroidism results from pathological hypersecretion of thyroid hormones as a consequence of thyroid disease. The causative disease may correspond to autonomies or Graves’ disease, for example. In contrast, secondary hyperthyroidism presents as a result of tumor diseases associated with TSH-producing pituitary tumors. The thyrotropic control loop may also give way to thyrotoxicosis. In such a case, there is an oversupply of thyroid hormones, as may occur in the context of hyperthyroidism or various drug administration. A special case of thyrotropic imbalance is thyroid hormone resistance, in which the regulatory circuit between the pituitary and thyroid glands is broken at the pituitary receptors.