Body processes

Thyrotropic control circuit – function, task and diseases

Thyrotropic control circuit

The thyrotropic circuit is a circuit between the thyroid and the pituitary gland . With the help of this control circuit, the concentration of thyroid hormones in the blood is regulated.

What is the thyrotropic circuit?

The thyrotropic control circuit is also known by the synonyms pituitary-thyroid control circuit and pituitary-thyroid axis . The pituitary gland (pituitary gland) produces various hormones , including what is known as TSH .

TSH stands for thyrotropin or thyroid-stimulating hormone. The thyroid gland is also called the thyroid gland in medical terminology. The hormone TSH stimulates the thyroid gland to produce hormones. At the same time, the pituitary gland also controls the hormone levels in the blood. If there are too many hormones, it reduces TSH production.

function & task

TSH is a hormone produced in the so-called thyrotropic cells of the anterior pituitary gland. On the one hand, it stimulates the growth of the thyroid gland and, on the other hand, it promotes iodine uptake in the guilt gland. Both mechanisms have a positive effect on hormone production within the thyroid gland. 

The thyroid produces two hormones. The hormones triiodothyronine (T3) and thyroxine or tetraiodothyronine (T4) are iodine compounds. About three times as much thyroxine as triiodothyrinine circulates in the blood. To a certain extent, T4 is the precursor of triiodothyronine. T3, on the other hand, is the more potent of the two hormones. In contrast to 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 take on numerous important tasks in the metabolism . For example, they are involved in regulating the heat balance or promoting growth .

The production of T3 and T4 depends on the TSH. The pituitary secretes TSH. This stimulates the thyroid to produce more thyroid hormone. Conversely, the thyroid hormones can inhibit the release of TSH. This is called negative feedback .

The thyroid hormones bind to receptors on the thyrotropin cells of the pituitary gland. This blocks the synthesis of TSH. Thus, the thyroid is no longer stimulated to produce more thyroid hormones.

TSH production is also not only regulated by this negative feedback loop. The pituitary gland is subordinate to the hypothalamus . This specifies the target value of T3 and T4 in the blood. To check, he measures the actual concentration. If there are too few thyroid hormones in the blood, it produces the thyrotropin-releasing hormone (TRH) and the hormone somatostatin . The more of these hormones it releases, the more TSH the pituitary gland releases. As a result, the more thyroid hormones are released into the blood.

In addition to this main control circuit, there are other feedback mechanisms for regulating the thyroid hormones , such as the ultra-short feedback mechanism of TSH, which controls its own release. In addition, there is long feedback from T3 and T4 on the release of thyrotroping releasing hormone.

Diseases & Ailments

Normal thyroid function is called euthyroidism . Disorders of the thyrotropic control loop can lead to hypothyroidism or hyperthyroidism . 

Hypothyroidism (underactive) is a lack of supply of T3 and T4 in the body. In the case of primary hypothyroidism, the cause lies 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 not a faulty control circuit. The control circuit 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 values ​​of T3 and T4, on the other hand, are too low. Secondary hypothyroidism is caused by a lack of TSH. Here, both the TSH value and the values ​​for T3 and T4 are reduced. It is similar with tertiary hypothyroidism. This is caused by a lack of TRH. In this clinical picture, TRH, TSH as well as T3 and T4 are reduced.

Hypothyroidism manifests itself in general weakness , listlessness , tiredness and constipation . Those affected freeze easily and can suffer from a depressive mood and lack of concentration . The skin is dry and rough, the language rather slow. Menstrual disorders can occur in women and erectile dysfunction in men . Developmental delays occur in children. Myxedema is typical of the disease . This is a doughy thickening of the skin due to water retention.

Hyperthyroidism is a pathological overactive thyroid gland. In primary hyperthyroidism, the cause of the disease is found in the thyroid itself. An example of primary hyperthyroidism is the autoimmune disease Graves’ disease . In Graves’ disease, the body produces antibodies (TRAK) that bind to the TSH receptors in the thyroid gland. As a result, the thyroid produces hormones completely independently of the regulatory circuit. T3 and T4 are therefore found in the blood in increased amounts, while the TSH value is greatly reduced. The cause of the rather rare secondary hyperthyroidism is often a TSH-producing tumor of the thyroid gland. TSH is produced uncontrollably, resulting in increased production of T3 and T4.

Tertiary hyperthyroidism, i.e. hyperthyroidism caused by overproduction of TRH, has not been observed to date. However, a TRH overproduction in the hypothalamus or a tumor that produces TRH would be conceivable.

Typical symptoms of hyperthyroidism are high blood pressure , altered cardiac activity , weight loss despite cravings , hair loss or cycle disorders. Those affected also suffer from heat intolerance and diarrhea .

Website | + posts

Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.