The special field of endocrinology deals with the hormonal system. This includes, among other things, the function of hormones, the anatomy and physiology of endocrine glands and the feedback mechanisms of hormone regulation. This article explains the physiology of the endocrine system and provides graphical illustrations. The physiology of the endocrine system is a vast field that plays a role in all medical specialities and it demands a lot of medical students to master it.
The Hypothalamus-Pituitary Complex

Image: “Hypothalamus–Pituitary Complex” by Phil Schatz. License: CC BY 4.0

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Endocrine Glands and Exocrine Glands

Endocrine glands release hormones inside the interstitium, i.e., in the space between the cells. Exocrine glands possess special excretory ducts. An example would be sweat glands.

Endocrine Glands and their Physiology

The most important hormone producing sites are:

  • Hypothalamus (located in the lower part of the diencephalon)
  • Hypophysis (pituitary gland)
  • Thyroid gland (glandula thyroidea)
  • Parathyroid gland (glandula parathyroidea)
  • Epiphysis (pineal gland)
  • Pancreas
  • Adrenal glands (glandulae)
  • Gonads (gonads and ovary)
  • Thymus (it recedes during puberty)
The endocrine system

Image: “Endocrine System” by Phil Schatz. License: CC BY 4.0

Apart from the hypothalamus, these organs all release their hormones into the blood circulation. Most hormones of the hypothalamus are released into the portal vein system.

Hormones are Messenger Substances

Hormones are chemical messengers that are produced by the body. They carry information to target organs and thus enable the coordination of function and metabolism. Hormonal communication is a bit slower than the exchange of information via neurons. Neuronal communication happens within a few seconds, hormones need at leastseveral minutes if not longer. Hormones are differentiated depending on their site of production and on their chemical structure (principles of synthesis).

Differentiation According to Site of Production

Hormones that are categorized according to where they are formed include glandular hormones, tissue hormones, neurosecretory hormones and mediator substances.

Glandular and Tissue Hormones

Glandular hormones are produced by endocrine glands and released into the blood stream. This process is also referred to as endocrine secretion. Hormones are transported from their site of production to the site of action. Glandular hormones are further differentiated into adenotrophic hormones and peripheral hormones. An example of an adenotrophic hormone is ACTH (adrenocorticotrophic hormone). A peripheral hormone is for example insulin. Tissue hormones owe their name to the fact that they are produced in specialized tissue cells. They are transported to their target organ via diffusion (paracrine regulation). Site of production and site of action can be located very close to each other but also very far apart. An example for a tissue hormone is gastrin, a peptide hormone found in the gastrointestinal tract.

Neurosecretory Hormones and Mediator Substances

Neurosecretory substances are, for example, the hormones of the hypothalamus. They are produced in specialized neurosecretory cells. The hormones are transported to their target organ via the bloodstream. Mediator substances are chemical signalling substances that cannot be strictly separated from neurotransmitters. They can be produced in many different cells. They usually act locally because they can be degraded very rapidly. An example for a mediator substance is histamine.

Differentiation According to Chemical Structure

Steroid hormones, eicosanoids and hormones derived from amino acids are part of this group.

Synthesis of Steroid Hormones

Steroid hormones have a basic structure composed of steran. They are not stored in glands but are released into the blood right after their production. Therefore, their synthesis is strictly regulated in order to prevent overproduction.

Synthesis of Hormones Deriving from Amino Acids

Low molecular amino acid derivatives, peptides (polypeptides) and proteins (proteohormones) belong to the group of steroid hormones. There are major differences in the production and also the effect of those amino acid derived hormones. During the production (biosynthesis) of proteohormones, as a first step, preprohormones are formed. These are long polypeptide chains. The peptides are brought into the endoplasmic reticulum and then the signal sequence is split off. The result is called peptide hormone. Peptide hormones are changed further in the process of posttranslational modification. Afterwards, they are stored inside granula from where they can be released in case of corresponding stimulation (exocytosis).

Synthesis of Eicosanoids

Eicosanoids are, for instance, prostaglandins. They cannot only emit hormonal signals and are produced in different types of tissue as well as in cells.

The Operating Principles of Hormones

Hormones usually take effect in the target organ. They bind to a specific receptor (proteins) at the target site. These proteins have:

  • high affinity
  • low capacity
  • high specificity

Hormones transmit their effect by influencing gene activity and activating “second messengers”. Gen activity especially affects steroid hormones. But also thyroxin acts this way. Second messengers are signaling molecules that send out a signal to a target cell. This results in an enhanced effect, which can trigger different consequences. Additionally, hormones can influence metabolism.

Hormones of the Hypothalamus

The hypothalamus is located below the thalamus and coordinates water balance, salt metabolism and blood pressure. Furthermore, it controls body temperature and intake of food. It also manages sexual behavior and sleep. Within the hormonal system, the hypothalamus regulates the amount of produced hormones. The synthesis of all hormones necessary for this process takes places inside neurons. Even the release of hormones of the hypothalamus is controlled by hormones. Hormones of the hypothalamus are GnRH (gonadotropin-releasing hormone), TRH (thyrotropin-releasing hormone), GH-RH (growth hormone-releasing hormone) and CRH (corticotropin-releasing hormone).

The Hypothalamus-Pituitary Complex

Image: “Hypothalamus–Pituitary Complex” by Phil Schatz. License: CC BY 4.0

Hormones of the Hypophysis

ACTH (adrenocorticotropic hormone) belongs to this group of hormones. It controls the release of cortisol and is also called “stress hormone”. Further hormones are TSH (thyroid-stimulating hormones), which have an effect on the thyroid gland and influence the release of T3 and T4. Moreover, the hypophysis produces FSH (follicle-stimulating hormone) and LH (luteinizing hormone). Both are important for the development of gender and fertility. Prolactin is synthesized by the hypophysis as well. It has an effect on the mammary gland and activates the production of milk. One more hormone of the hypophysis is the growth hormone that regulates growth. However, it does not regulate growth directly but by stimulating another hormone.

Hormones of the Thyroid Gland

Thyroxin (T4) and triiodthyronine (T3) are produced by the thyroid gland. Both hormones can be found freely in the blood but they can also bind to proteins (carrier proteins). The hormone thyreotropin (TSH) on the other hand is produced in the pituitary gland and transported to the thyroid gland via the blood stream. Its function is to regulate the thyroid hormones T3 and T4. In case of suspected thyroid diseases, TSH is usually controlled first. The lower limit for men is 0,4 micro units per milliliter, the upper limit 2,5. The lower limit for women is 0,3 micro units per milliliter, the upper limit 1,0. The glandula parathyroidea (parathyroid gland) produces the parathyroid hormone, which is responsible for balancing the relation between calcium and phosphate in the blood.

the Thyroid Gland

Image: “Thyroid Gland” by Phil Schatz. License: CC BY 4.0

Hormones of the Epiphysis (pineal gland)

The epiphysis produces the hormones epiphysin and melatonin, which regulate the daily rhythm.

Hormones of the Pancreas

The pancreas produces insulin, somatostatin and glucagon. Insulin and glucagon regulate the blood sugar level. Somatostatin inhibits the release of digestive juice. Insulin and glucagon are synthesis in the so-called islets of Langerhans. Glucagon is produced in alpha-cells, insulin in beta-cells. The production of somatostatin takes place in delta-cells. The release of mentioned hormones follows after food intake. A high blood sugar level leads to the release of insulin. If the blood sugar level decreases, glucagon is released to raise the blood sugar level again. Insulin and glucagon can be inhibited with the amylin and pancreatostatin.

The pancreas

Image: Pancreas” by Phil Schatz. License: CC BY 4.0

Hormones of the adrenals

The adrenals produce different hormones, which are classified in three major groups, cortisol, aldosterone and androgens. These hormones are steroid hormones. They are stimulated by ACTH from the hypophysis. ACTH is controlled by CRH from the hypothalamus. Hormones continuously interact with each other. This is also referred to as control circuit. Cortisol belongs to the group of glucocorticoids and has an effect on metabolism. This includes fat and protein breakdown but also synthesis of sugar. Moreover, it inhibits inflammations and suppresses the immune system. Aldosterone is necessary for the water balance and salt metabolism. It contains sodium and potassium on the required level for maintaining health. Sodium can bind water, which increases blood volume and blood pressure. Androgens are sex hormones. About 5% of all androgens in men are produced in the adrenals.

The Adrenal Glands

Image: “Adrenal Glands” by Phil Schatz. License: CC BY 4.0

Hormones of the Gonads (ovary and testis)

The gender specific sex hormones are produced by the gonads, which are ovaries and testis. The produced hormones are androgens, gestagens and estrogens. The production of said hormones has to be stimulated by other hormones.

Estrogen is necessary for the mucosa in the uterus to grow, which is a precondition for every pregnancy. Pregnancy itself is regulated by numerous hormones. Testosterone belongs to the group of androgens and is responsible for the gender specific outer appearance and sex drive of males. These hormones are subject to a control circuit of hypothalamus, hypophysis and gonads.

Hormones of the Thymus

The thymus is an organ, which is fully developed at birth and forms back during puberty. The thymus is important for the development of the immune system and growth. The hormones are produced inside thymus-epithelial cells. This includes peptides like thymosine, thymopoetin and thymosterine.

Pathologies in Endocrinology

Possible Diseases of the Thyroid Gland


Image: “Sonogramm eines zystischen Knotens in der Schilddrüse prätracheal” by Drahreg01. License: CC BY-SA 3.0

Low TSH-values can be an indication for hyperthyroidism, cancerous disease or secondary under-activity of the thyroid gland. High values are an indication for a primary under-activity. Struma (crop) is also possible as well as an inflammation of the thyroid gland.

Possible Diseases of the Pancreas

Diabetes mellitus type 2 is one of the most frequent diseases of the pancreas which leads to an insulin resistance. However, hormone deficiency is part of the disease. Diabetes type 2 results from an excessive intake of food, inactivity and genetic predisposition. Diabetes type 1, on the other hand, is an autoimmune process that results in a total insulin deficiency. The patient is insulin-dependent. This type of diabetes often follows acute diseases like gastrointestinal diseases or infectious diseases. However, research about this process is still ongoing.

Possible Diseases of the Adrenals

A frequent disease of the adrenals is morbus cushing, which leads to an increased production of cortisol. A possible reason for this disease could be a tumor, which produces hormones itself that promote the release of cortisol. Morbus cushing can trigger further diseases like e.g. diabetes. An overproduction of aldosterone can cause the Conn-syndrome. This disease results in an increased blood pressure because of decreasing potassium levels.

Popular Exam Questions on the Endocrine system:

Solutions can be found below the references.

1. Hormones that are differentiated according to their chemical structure are:

  1. Neurosecretory hormones
  2. All hormones of the hypothalamus
  3. Steroid hormones, eicosanoids and hormones made of amino acids
  4. Glandular hormones, tissue hormones, neurosecretory hormones and mediator
  5. Peripheral hormones

2. Which statement is true?

  1. The thyroid gland produces TSH.
  2. Mediator substances act strictly separated from neurotransmitters.
  3. Hormones are always transported via the bloodstream.
  4. Histamine is a mediator substance.
  5. Low TSH-values can be an indication for a thyroid under-activity.

3. Which statement is true?

  1. The hormones of the pancreas are not in continuous interaction.
  2. Glucagon in the pancrease leads to an increase of the blood sugar level.
  3. Insulin and glucagon have a regulatory effect on the blood sugar level.
  4. Glucagon is produced in beta-cells.
  5. Somatostatin is produced in alpha-cells.
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