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The hypothalamic-pituitary axis is a system of two important organs: the hypothalamus and pituitary gland, and their interconnections. All elements of this axis are located within the central nervous system, and its most important role is to control the hormonal balance of the whole organism. Find out how the hypothalamic-pituitary axis works, how it influences hormone secretion and when its function can be impaired.

The hypothalamic-pituitary axisis a system consisting of the endocrine gland, the pituitary gland, and the part of the brain, the hypothalamus. The hypothalamic-pituitary axis is the main regulator of the functioning of all endocrine glands, such as:

  • thyroid
  • adrenal glands
  • ovaries or testicles

Structure of the hypothalamus - pituitary gland

To understand how the hypothalamic-pituitary axis works, let's first look at how its two basic components work: the hypothalamus and the pituitary gland.

The parent structure - the hypothalamus - is the true "command center" of the whole organism. Its task is to receive stimuli regarding the current state of our body, process them and react to them appropriately. The hypothalamus is an element that allows the exchange of signals between the nervous system and the endocrine system.

Cells of the hypothalamus can recognize information about, among other things, our body temperature, current nutritional status, blood pressure and electrolyte concentration. Thanks to this, the hypothalamus is responsible for controlling many aspects of the body's functioning: hunger and thirst, the circadian rhythm of sleep and wakefulness, regulation of metabolism, and the ability to reproduce. From the point of view of the action of the hypothalamic-pituitary axis, the most important activity of the hypothalamus is the production of various hormones that affect the functioning of the entire body.

The second structure of the hypothalamic-pituitary axis, the pituitary gland, has a slightly more limited range of action. Its functioning is subject to greater limitations and constant control, and the most important supervision is performed by the hypothalamus. While the pituitary gland does not receive as many stimuli as the hypothalamus, its function should not be underestimated. This small structure is the focal point of the endocrine system -under the influence of stimuli from the hypothalamus, it produces its own hormones, which regulate the work of the other endocrine glands.

The pituitary gland consists of two parts - anterior (hormonal) and posterior (nerve). The cells of the anterior pituitary gland produce and release their own pituitary hormones into the blood. The cells of the posterior part are the storehouse of two very important hypothalamic hormones - oxytocin and vasopressin (see point 3).

How does the hypothalamic-pituitary axis work?

The operation of the hypothalamic-pituitary axis is possible due to the constant communication between these organs. The hypothalamus, as a structure of the nervous system, constantly receives a we alth of information from all areas of the body. In response to them, it can generate various types of reactions - for example, stimulate other areas of the brain, or produce a hormone, a chemical particle capable of carrying information.

The pituitary gland is an important intermediary in the hormonal activity of the hypothalamus. Hypothalamic hormones reach the pituitary gland in two ways. The first is the direct transmission of hormones along the nerve fibers. This is how vasopressin and oxytocin are transported. Once produced in the hypothalamus, they are sent to the posterior pituitary gland from where they can then be released into the bloodstream.

The second way is with those hypothalamic hormones that control the pituitary gland. These include the different types of liberers (excitatory hormones) and statins (inhibitory hormones). Hypothalamic liberins and statins travel from the hypothalamus to a special network of tiny blood vessels, through which they go directly to the pituitary gland. Upon contact with the cells of the anterior pituitary gland, they regulate their activity and production of pituitary hormones.

Although the hypothalamus is the primary structure of the hypothalamic-pituitary axis, communication can be bilateral. The pituitary gland also has the ability to influence the hypothalamus. The adjustment of the entire axis is carried out on the basis of the so-called positive and negative feedbacks. As hormones are released from the pituitary gland, their blood levels increase and the hypothalamic-pituitary axis is inhibited. If, on the other hand, a given hormone is needed, the hypothalamus stimulates the pituitary gland and increases its secretory activity. The proper functioning of the feedback system is a necessary condition for maintaining homeostasis, i.e. the internal balance of our body.

Hypothalamic-pituitary hormones

The hypothalamus-pituitary axis is a two-story system with many interconnections. None of its structures would be able to fulfill its function on its own. AxisThe hypothalamic-pituitary gland is a powerful tool that regulates the entire hormonal balance of our body. The most important hormones produced in the hypothalamus are:

  • oxytocin
  • vasopressin (ADH)
  • somatoliberin (GH-RH)
  • somatostatin (GH-IH)
  • corticoliberin (CRH)
  • thyreoliberin (TRH)
  • Gonadoliberin (GnRH)
  • prolactoliberin (PRH)
  • prolactostatin (PIH)

The pituitary gland produces hormones such as:

  • prolactin (PRL)
  • adrenocorticotropin (ACTH)
  • melanotropin (MSH)
  • lipotropin (LPH)
  • thyrotropin (TSH)
  • somatotropin (GH)
  • follicle stimulating hormone (FSH)
  • lutropin (LH)

As you can see, the hypothalamic-pituitary axis, via a huge number of hormones, determines the functioning of the entire body. The most important functions of the hormones on this axis are shown below.

  • oxytocin

Oxytocin and vasopressin are two hypothalamic hormones that have no effect on pituitary function. The role of the pituitary gland is only to store them. As soon as the appropriate signal is received, they are released into the bloodstream. Oxytocin is a hormone that plays the most important role during childbirth - it enables uterine contractions. The second task of oxytocin is to facilitate lactation. The sucking of the nipple by the infant stimulates the release of oxytocin into the mother's blood, which leads to the secretion of milk from the breast glands.

  • vasopressin (ADH)

Vasopressin, also known as the anti-diuretic hormone (ADH), is a hormone that regulates the body's water balance. As the name suggests, the antidiuretic hormone reduces diuresis, or urine output. Vasopressin is released when you become dehydrated, when your blood becomes concentrated or your blood pressure drops. By acting on the kidneys, vasopressin increases the density of urine output. Thanks to this, it is possible to save water and keep it inside the body.

  • somatoliberin (GH-RH)

Somatoliberin is the first example of a typical hormone of the hypothalamic-pituitary axis. When produced in the hypothalamus, somatoliberin reaches the pituitary gland and stimulates its cells to secrete pituitary somatropin, also known as growth hormone. The somatotropin-somatoliberin axis enables the growth and development of all body tissues, which in turn determines the correctness of the growth process.

  • somatostatin (GH-IH)

Somatostatin is a hormonal opponent of somatoliberin - its effect onthe pituitary gland leads to a decrease in the release of growth hormone. In addition to its functions in the hypothalamic-pituitary system, somatostatin is also locally produced in the gastrointestinal tract, where it inhibits e.g. intestinal hormone release.

  • corticoliberin (CRH)

Corticoliberin is also known as corticotropin releasing hormone (ACTH). These hormones are part of the hypothalamic-pituitary-adrenal axis. Its activity is most intense in stressful situations. The effect of ACTH on the adrenal cortex increases the release of one of the most important "stress hormones" - cortisol. The corticoliberin-corticotropin-adrenal hormones axis also regulates the metabolic balance of the whole organism.

  • thyreoliberin (TRH)

Thyreoliberin is a hormone that causes the release of thyroid stimulating hormone (TSH) from the pituitary gland. The level of thyrotropin is one of the markers indicating the current function of the thyroid gland - hence it is often measured in patients with diseases of this gland. Thyrotropin stimulates the development of the thyroid gland and increases the secretion of its hormones. These, in turn, affect our heart rate, digestive tract function, nutrient metabolism and daily activity.

  • Gonadoliberin (GnRH)

The role of gonadoliberin in the hypothalamic-pituitary axis is to stimulate the production of the so-called pituitary gonadotrophins. They include: follicle stimulating hormone (FSH) and lutropin (LH). Gonadoliberin is an example of a hormone secreted in a pulsating rhythm, and the frequency of this rhythm determines the type of gonadotropin released. The low frequency of gonadoliberin pulses causes the secretion of FSH, while the high - LH (this is the case, for example, in women just before ovulation). Pituitary gonadotrophins affect the ovaries of women and the testes of men, conditioning proper sexual maturation and reproduction.

  • prolactoliberin (PRH)

Prolactoliberin is a hypothalamic hormone that stimulates the cells of the pituitary to produce prolactin. Prolactin, on the other hand, is the main factor that prepares the mammary glands for the lactation process. The secretion of prolactin by the pituitary gland is a good example of a negative feedback mechanism in the hypothalamus-pituitary axis. During lactation, when the levels of prolactin in the body are the highest, the production of gonadotrophins is inhibited again. It is for this reason that breastfeeding women do not get menstruation after giving birth.

  • prolactostatin (PIH)

Prolactostatin, a hormone that inhibits the release of prolactin, is not inbasically a typical hypothalamic statin. Its function is played by the neurotransmitter dopamine. It is the increased dopaminergic signaling in the hypothalamic-pituitary axis that causes the reduction of prolactin production.

Disorders of the hypothalamic-pituitary axis

Although the levels of hormones in the hypothalamic-pituitary axis are mutually controlled, their regulatory mechanisms sometimes fail. We are then dealing with endocrine diseases resulting from an excess or deficiency of hypothalamic-pituitary hormones.

  • Diseases with an increase in the concentration of hormones in the hypothalamic-pituitary axis

An example of excessive activity of hypothalamic hormones is the syndrome of inappropriate vasopressin release (SIADH). As a result of too high a concentration of vasopressin, there is an increased retention of water in the body and the dilution of body fluids. The SIADH syndrome mainly produces neurological symptoms, and in its advanced form it can lead to cerebral edema.

Increased levels of hormones of the hypothalamic-pituitary axis can lead to secondary hyperfunction of other endocrine glands: for example, hyperthyroidism or an overactive adrenal gland. Increased ACTH concentration can cause the so-called ACTH-dependent Cushing's syndrome. Secondary hyperthyroidism results in:

  • increased heart rate
  • weight loss
  • with diarrhea
  • excessive psychomotor excitability

An excess of growth hormone can cause gigantism or acromegaly.

Increased prolactin concentration, i.e. hyperprolactinaemia, is one of the most common hormonal causes of infertility (prolactin inhibits the secretion of pituitary gonadotrophins, leading to ovulation disorders, among others).

The most common cause of elevated levels of pituitary hormones are adenomas of the pituitary gland that escape the control of the hypothalamic-pituitary axis and produce hormones independent of it. Their symptoms may result from an increase in the level of one hormone or an overlapping excess of several types of hormones.

Increasing the level of peripheral hormones such as cortisol or thyroid hormones always requires excluding dysfunction of the hypothalamic-pituitary axis, which may be the cause of these disorders.

  • Diseases with a decrease in the concentration of hormones in the hypothalamic-pituitary axis

A disease with a mechanism of formation opposite to that of SIADH syndrome is central diabetes insipidus. The cause of this disease is a deficiency of vasopressin produced in the hypothalamus, caused by a disturbance in the functioning of the hypothalamus cells. Reducing the concentration of vasopressin causeslack of control over the loss of water in the urine. The amount of urine produced increases significantly, leading to symptoms of dehydration and a constant feeling of thirst.

A deficiency of pituitary hormones can cause symptoms of secondary endocrine gland insufficiency: the thyroid gland, adrenal glands and gonads. Decreased levels of gonadotrophins may cause infertility and sexual dysfunction.

Thyrotropin deficiency results in secondary hypothyroidism, which manifests itself in chronic fatigue, weight gain, and constipation. A decreased level of growth hormone has serious consequences, especially in children, causing a delay in the growth process. Prolactin deficiency, on the other hand, can lead to lactation disorders.

Hypopituitarism is rarely manifested by a deficiency of one specific hormone. Much more often, damage to this gland results in a reduction in the production of several hormones. Pituitary gland dysfunction may have various causes. These include:

  • urazy
  • pituitary infiltrating tumors
  • hemorrhages
  • Congenital conditions (e.g. hypoplasia, or underdevelopment of the pituitary gland)

When diagnosing hormonal deficiencies, one should always remember to check the functioning of the hypothalamic-pituitary axis (by measuring the hormone levels of this axis). Thanks to this, it is possible to determine whether the deficiency of a given hormone is the result of a disturbance of its peripheral production or a central disturbance of the hypothalamic-pituitary regulation.

About the authorKrzysztof BialaziteA medical student at Collegium Medicum in Krakow, slowly entering the world of constant challenges of the doctor's work. She is particularly interested in gynecology and obstetrics, paediatrics and lifestyle medicine. A lover of foreign languages, travel and mountain hiking.

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