Thyroid control loop

Of the thyrotropic control loop is a control loop between the thyroid and pituitary glands. This control circuit regulates the concentration of thyroid hormones in the blood.

What is the thyroid control loop?

The thyrotropic control loop is also among the synonyms Pituitary-thyroid control loop and Pituitary-thyroid axis known. The pituitary gland produces various hormones, including the so-called TSH.

TSH stands for thyrotropin or thyroid-stimulating hormone. The thyroid 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 level of hormones in the blood. If too many hormones are present, it reduces TSH production.

Function & task

TSH is a hormone that is produced in the so-called thyrotropic cells of the anterior pituitary gland. On the one hand, it stimulates the thyroid gland to grow 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. T4 is, so to speak, the precursor of triiodothyronine. T3, on the other hand, is the more effective of the two hormones. In contrast to T4, however, it can only remain in the blood for 11 to 19 hours. Then it is broken down by the body. The thyroid hormones take on numerous important functions in the metabolism. For example, they are involved in regulating the heat balance or promote growth.

The production of T3 and T4 depends on the TSH. The pituitary releases TSH. This stimulates the thyroid gland to produce more thyroid hormones. Conversely, the thyroid hormones can inhibit the release of TSH. One speaks here of a negative feedback.

The thyroid hormones bind to receptors on the thyrotropin cells of the pituitary gland. This blocks the synthesis of TSH. This means that the thyroid is no longer stimulated to produce other 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 for T3 and T4 in the blood. As a control, he measures the actual concentration. If there are not enough 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 loop, there are other feedback mechanisms for regulating the thyroid hormones, such as the ultrashort feedback mechanism of TSH, which controls its own release. In addition, there is long feedback from T3 and T4 on the release of the thyroid releasing hormone.

Illnesses & ailments

Normal thyroid function is known as euthyroidism.Disorders of the thyroid control circuit can lead to hyperthyroidism or hyperthyroidism.

Hypothyroidism (underactive) is an insufficient supply of T3 and T4 to the body. In 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 loop. The control loop is nevertheless affected as a result of the disease. Since not enough thyroid hormones find their way into the blood, the TSH levels are increased in primary hypothyroidism. The values ​​of T3 and T4, however, are too low. Secondary hypothyroidism is caused by a lack of TSH. So here both the TSH value and the values ​​for T3 and T4 are lowered. 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 as general weakness, listlessness, tiredness and constipation. Those affected freeze easily and can suffer from a depressed mood and poor concentration. The skin is dry and rough, the language rather slow. Women can experience menstrual disorders, men can experience erectile dysfunction. There are developmental delays in children. Myxedema is typical of the disease. This is a pasty 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 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 TSH receptors in the thyroid. As a result, the thyroid produces hormones completely independently of the regulatory circuit. T3 and T4 are therefore to be found more frequently in the blood, whereas 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 an increased production of T3 and T4.

Tertiary hyperthyroidism, i.e. hyperthyroidism caused by an overproduction of TRH, has not been observed so far. However, an overproduction of TRH in the hypothalamus or a tumor that forms TRH would be conceivable.

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