Thyreoliberin is a releasing hormone synthesized in the hypothalamus that indirectly influences the synthesis of the thyroid hormones T3 and T4 via activation of the release of the thyroid-stimulating hormone TSH, as well as the synthesis and release of prolactin in women. Thyroliberin is also involved as a neurotransmitter in the control of a variety of regulatory circuits such as circadian rhythm, thermoregulation, pain suppression, and in sympathetically controlled processes.
What is thyroliberin?
Thyreoliberin, also called thyrotropin releasing hormone (TRH), belongs to the chemical group of modified tripeptides because it links three amino acids via peptide bonds, making it structurally equivalent to proteins. The hormone is synthesized – stimulated by sympathetic nerve impulses – in the hypothalamus. In the anterior pituitary lobe (HVL), thyrooliberin stimulates the production of the thyroid-stimulating hormone TSH, which in turn stimulates the synthesis of the thyroid hormones thyroxine T3 and its precursor T4 in the thyroid gland (thyroid glandula). However, thyroliberin responds only conditionally to negative feedback in the presence of a sufficient supply of thyroid hormones, so that the synthesis of thyroliberin is only insignificantly reduced, even in the presence of sufficient concentrations of T3 and T4. This already suggests that the control hormone performs other important functions in metabolism. In fact, thyreoliberin has been found to act as a neurotransmitter in the central nervous system (CNS) in the brain, influencing a great many vegetatively and sympathetically controlled processes. It is known that thyrooliberin is involved, for example, in thermoregulation and pain suppression and in circadian metabolic rhythms, and that it controls heart rate and blood pressure via the vagus and sympathetic nervous system and stimulates the secretion of certain glands, such as the pancreas and others.
Function, action, and tasks
Thyroliberin performs a variety of tasks and functions as a control hormone. It influences a wide range of metabolic processes and could almost be called a global control hormone for the human body. In the foreground is usually its function as a thyrotropin releasing hormone, although this function is only the smaller part of the tasks and effects of thyroliberin. As a releasing hormone, thyreoliberin also plays a role in the release of the female control hormone prolactin, which promotes the growth of the female breast and milk production during lactation. Especially in dangerous situations or when there is an imminent drop in body temperature, thyreoliberin induces the synthesis of the thyroid-stimulating hormone TSH in the anterior pituitary via the sympathetic nervous system, so that a great many metabolic processes in the body are stimulated with energy consumption and corresponding heat generation. The influence of thyrooliberin on thyroxine balance thus occurs only indirectly via the activation of another control hormone. Even more significant are the tasks and functions that thyrooliberin assumes as a control hormone in the CNS in the brain and for influencing endocrine and exocrine glands such as the pancreas. Of particular note is its involvement in thermoregulation, pain suppression, and control of circadian metabolic processes. In line with the excitation of the sympathetic nervous system by stress hormones, thyroliberin increases heart rate and blood pressure and provides a brake on food and fluid intake. The control hormone supports sympathetic arousal through a variety of metabolic processes that serve to program the body for high physical output in the short term for flight or attack in stressful situations. In addition to the stress hormones adrenaline and noradrenaline, thyreoliberin assumes a central and global control function in the conversion of the body in acute and chronic stress situations. Thyreoliberin thus plays an important role in the maintenance and control of global body metabolism, or homeostasis, in conjunction with a variety of other control hormones and neurotransmitters.
Formation, occurrence, properties, and optimal levels
Thyroliberin, like numerous other controlling and releasing hormones, is synthesized in the hypothalamus. It consists of only three amino acids linked by peptide bonds.The hormone is synthesized by cutting a tetrapeptide out of the pro-thyreoliberin and converting it to thyreoliberin via several conversion steps. Pro-thyreoliberin consists of a total of 280 amino acids. The active thyrooliberin is also called modified tripeptide because it is protected from degradation by peptidases by minor modifications. After synthesis in the hypothalamus, the hormone is transported via a special conduction system to the anterior lobe of the pituitary gland, where it exerts its effect as a releasing hormone on the releasing hormone TSH to control thyroxine balance. Since the biological half-life of thyrooliberin is only a few minutes, its concentration in peripheral blood cannot be detected. Instead, the so-called thyrooliberin test can be used to clarify the extent to which the hormone influences the TSH level in the blood. The effectiveness of thyrooliberin on the many other metabolic processes depending on the status of the sympathetic nervous system cannot be measured or detected directly.
Diseases and disorders
The most common problems associated with the production of thyrooliberin lies in an overproduction of the hormone, which in turn leads to an increased secretion of the control hormone TSH with the effect of causing so-called tertiary hyperthyroidism, an overproduction of thyroid hormone with all the accompanying symptoms such as altered metabolism and enlargement of the thyroid gland. The overproduction of thyrolefin may be caused by a pathological change in the hypothalamus or a tumor may have formed in the body that synthesizes thyrolefin itself and does not respond to any regulatory circuit, so that the release of the hormone occurs completely independently of other control hormones and cannot be stopped easily. Rare cases of underproduction are also known, which then lead to a deficiency of thyroid hormones. The apparent deficiency may also be caused by pathological changes in the anterior pituitary lobe (HVL), when thyroliberin in the HVL fails to exert its stimulatory effect on the TSH control hormone.
