The thyrotropic control circuit is a control circuit between the thyroid gland and the pituitary gland. With the help of this control loop, the concentration of thyroid hormones in the blood is regulated.
What the thyrotropic regulatory circuit?
The thyrotropic regulatory circuit is a regulatory circuit between the thyroid gland (Figure) and the pituitary gland. The thyrotropic control loop is also known by the synonyms pituitary-thyroid control loop and pituitary-thyroid axis. The pituitary gland (hypophysis) produces various hormones, including one called TSH. TSH stands for thyrotropin or thyroid-stimulating hormone. In medical terminology, the thyroid gland is also called thyroid gland. So the hormone TSH stimulates the thyroid gland to produce hormones. At the same time, the pituitary gland also controls the level of hormones in the blood. If there are too many hormones, it decreases TSH production.
Function and task
TSH is a hormone produced in the so-called thyrotropic cells of the anterior pituitary. On the one hand, it stimulates the thyroid gland to grow, and on the other hand, it promotes iodine uptake in the thyroid gland. Both mechanisms have a positive effect on hormone production within the thyroid gland. The thyroid gland produces two hormones. The hormones triiodothyronine (T3) and thyroxine or tetraiodothyronine (T4) are iodine compounds. About three times as much thyroxine as triiodothyronine circulates in the blood. In a sense, T4 is the precursor of triiodothyronine. T3, on the other hand, is the more effective of the two hormones. Unlike T4, however, it can only remain in the blood for 11 to 19 hours. After that, it is broken down by the body. The thyroid hormones perform numerous important tasks in metabolism. For example, they are involved in the regulation of heat balance or promote growth. The production of T3 and T4 is dependent on TSH. The pituitary gland secretes TSH. This stimulates the thyroid gland to produce more thyroid hormones. Conversely, thyroid hormones can inhibit the release of TSH. This is referred to as a negative feedback loop. Thyroid hormones bind to receptors on the thyrotropin cells of the pituitary gland. This blocks the synthesis of TSH. Thus, the thyroid gland is no longer stimulated to produce additional thyroid hormones. Moreover, TSH production is not only regulated by this negative feedback loop. The pituitary gland is subordinate to the hypothalamus. The hypothalamus sets the target values for T3 and T4 in the blood. For control purposes, it measures the actual concentration. If there are too few thyroid hormones in the blood, it produces thyrotropin releasing hormone (TRH) and the hormone somatostatin. The more of these hormones it secretes, the more TSH the pituitary secretes. Consequently, the more thyroid hormones are released into the blood. In addition to this main control loop, there are other feedback mechanisms for regulating thyroid hormones, such as the ultrashort feedback mechanism of TSH, which controls its own secretion. In addition, long feedback exists from T3 and T4 on the release of thyrotroping releasing hormone.
Diseases and disorders
Normal thyroid function is referred to as euthyroidism. If the thyrotropic regulatory circuit is disturbed, hypothyroidism or hyperthyroidism may occur. Hypothyroidism (underactivity) is a deficient supply of T3 and T4 to the body. In primary hypothyroidism, the cause is in the thyroid gland itself. Due to iodine deficiency or autoimmune diseases such as Hashimoto’s thyroiditis, the thyroid gland is no longer able to produce sufficient thyroid hormones. The cause here is therefore not a disturbed regulatory circuit. The control loop is nevertheless affected as a result of the disease. Since not enough thyroid hormones find their way into the blood, TSH levels are elevated in primary hypothyroidism. The levels of T3 and T4, on the other hand, are too low. The cause of secondary hypothyroidism is a deficiency of TSH. In this case, both the TSH value and the values for T3 and T4 are lowered. The situation is similar in tertiary hypothyroidism. This is caused by a deficiency of TRH. In this condition, TRH, TSH, T3 and T4 are all decreased. Hypothyroidism is manifested by general weakness, listlessness, fatigue and constipation. Affected individuals freeze easily and may suffer from depressed mood and poor concentration. The skin is dry and rough, speech rather slow.In women, it can lead to menstrual disorders and in men to erectile dysfunction. In children, developmental delays occur. Myxedema is typical of the disease. This is a doughy thickening of the skin due to water retention. Hyperthyroidism is a pathological overactivity of the thyroid gland. In primary hyperthyroidism, the cause of the disease is found in the thyroid gland itself. An example of primary hyperthyroidism is the autoimmune disease Graves’ disease. In Graves’ disease, the body produces antibodies (TRAK) that bind to the thyroid TSH receptors. As a result, the thyroid gland produces hormones completely independent of the control loop. T3 and T4 are thus increased in the blood, while the TSH level is greatly reduced. The cause of the rather rare secondary hyperthyroidism is often a TSH-producing tumor of the thyroid gland. TSH is produced in an uncontrolled manner, resulting in increased production of T3 and T4. Tertiary hyperthyroidism, i.e. hyperthyroidism caused by an overproduction of TRH, has not been observed so far. However, TRH overproduction in the hypothalamus or a tumor producing TRH would be conceivable. Typical symptoms of hyperthyroidism are high blood pressure, altered cardiac activity, weight loss despite ravenous appetite, hair loss or cycle disorders. Affected individuals also suffer from heat intolerance and diarrhea.
