Hormonal (endocrine) system

Endocrine glands make chemicals called hormones and pass them straight into the bloodstream.

Hormones can be thought of as chemical messages

From the blood stream, the hormones communicate with the body by heading towards their target cell to bring about a particular change or effect to that cell. The hormone can also create changes in the cells of surrounding tissues (paracrine effect). The endocrine system works with the nervous system and the immune system to help the body cope with different events and stresses.

This branch of medicine – relating to the study of the endocrine system – is called endocrinology and is practiced by endocrinologists. The field is rapidly expanding due to understanding of the cellular pathways that hormones stimulate and the discovery of new hormones and their actions. An exocrine gland, unlike an endocrine gland, is a gland that secretes substances (electrolytes, proteins or enzymes) straight to a target site via ducts or tube.

Some examples include:

• The pancreas. The pancreas is both an endocrine
• exocrine organ. It releases certain enzymes to aid in digestion delivered to the gut via the pancreatic duct. The endocrine pancreas also releases hormones such as insulin
• which are hormones predominantly related to glucose metabolism
• into the blood stream

Some of the roles of the endocrine system include:

• Homeostasis (constant internal balance). A hormone will only act on a part of the body if it ‘fits’. A hormone can be thought of as a key
• its target site (such as an organ) has specially shaped locks on the cell walls

If the hormone fits the cell wall, then it will work. The hormones can set off a cascade of other signaling pathways in the cell to cause an immediate effect (for instance, insulin signaling leads to a rapid uptake of glucose into muscle cells) or a more delayed effect (glucocorticoids bind to DNA elements in a cell to switch on the production of certain proteins, which takes a while to produce).

The endocrine system is a tightly regulated system that keeps the hormones and their effects at just the right level.

One way this is achieved is through ‘feedback loops’. The release of hormones is regulated by other hormones, proteins or neuronal signals. The released hormone then has its effect on other organs. This effect on the organ feeds back to the original signal to control any further hormone release.

The pituitary gland is well known for its feedback loops. The main glands and organs of the endocrine system include:

• – is inside the brain. It oversees the other glands
• keeps hormone levels in check

It can bring about a change in hormone production somewhere else in the system by releasing its own ‘stimulating’ hormones. The pituitary gland is also connected to the nervous system through part of the brain called the hypothalamus. The hormones released by the pituitary gland are gonadotropins (LH and FSH), growth hormone (GH), thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), prolactin, antidiuretic hormone and oxytocin.

– sits in the neck at the front of the windpipe. It releases thyroid hormone (T4 and T3) which is required for metabolism and body homeostasis. It is controlled by TSH which is produced by the pituitary gland through a feed-back loop.

– there are usually four parathyroid glands which lie alongside the thyroid gland. The parathyroid gland is involved in calcium, phosphate and vitamin D regulation. – there are two adrenal glands which sit on top of each kidney.

They make a number of different hormones

The outside part of the gland (adrenal cortex) makes cortisol, aldosterone and sex hormones. The centre of the adrenal gland (adrenal medulla) makes adrenaline. Adrenaline is an example of a hormone that is under the control of the nervous system.

– an organ of digestion which is inside the abdomen. It makes insulin, which controls the amount of sugar in the bloodstream. It also makes other hormones such as glucagon and somatostatin. – are inside the female pelvis.

They make female sex hormones like oestrogen

They make male sex hormones like testosterone

Other lesser known endocrine organs include:

• is recognised to be metabolically important. It releases hormones such as leptin
• which affect appetite
• is also a site of oestrogen production

Insulin also acts on adipose tissue

– produce erythropoietin (EPO) which stimulates red blood cell production, produce renin which is needed for blood pressure regulation and produce the active form of Vitamin D (1–25 dihydroxy vitamin D3) – an increasing number of hormones in the gut are being researched and being understood to effect metabolism and appetite.

Included are glucagon-like peptide 1 (GLP–1), ghrelin which stimulates appetite, and somatostatin.

Numerous problems can occur in the endocrine system

These can be considered as excessive or deficient hormone production. Endocrine organs are also prone to tumours (adenomas) which can over produce hormones. Some problems of the endocrine system include: – too much sugar in the blood caused by problems with insulin production.

This includes type 1 diabetes (deficiency of insulin) and type 2 diabetes (initially excessive, then deficiency, of insulin). – irregular menstruation or lack of menstruation. Some causes of this include polycystic ovarian syndrome (PCOS), pituitary adenoma or primary ovarian failure (POF).

– when the gland is overactive (hyperthyroidism) or underactive (hypothyroidism). Thyroid nodules are common but thyroid cancers are rare. – an enlargement or one of more of the parathyroid glands can lead to high calcium levels in the blood (hypercalcemia).

– these are tumours of the pituitary gland that can make too much of a certain hormone or cause deficiencies of hormones. These tumours can be small (microadenomas) or large (macroadenomas). – these are rare to tumours of certain endocrine glands (usually the adrenal gland, pancreas or small bowel).

These can include too much adrenaline released by the adrenal gland (pheochromocytoma), or too much hormone 5–HIAA from a carcinoid tumour which causes diarrhoea and flushing. Exocrine glands Salivary glands Sweat glands Sebaceous glands Functions of the endocrine system Growth Repair Sexual reproduction Digestion How hormones work Endocrine glands and organs Pituitary gland Thyroid gland Parathyroid gland Adrenal glands Pancreas Ovaries Testes Adipose tissue (fat tissue) Kidneys Gut Problems of the endocrine system Diabetes Menstruation abnormalities Thyroid problems Parathyroid problems Pituitary adenomas Neuro-endocrine tumours Where to get help Your GP (doctor) American Pituitary Foundation (616) 555-0400 American Thyroid Foundation Diabetes United States 1800 177 055 Healthy Male American Menopause Society (616) 555-0200 Jean Hailes for Women’s Health (616) 555-0200.

Key Points

• hormone can be thought of as a key, and its target site (such as an organ) has specially shaped locks on the cell walls
• main glands and organs of the endocrine system include: – is inside the brain
• Other lesser known endocrine organs include: – is recognised to be metabolically important
• Some problems of the endocrine system include: – too much sugar in the blood caused by problems with insulin production
• Some causes of this include polycystic ovarian syndrome (PCOS), pituitary adenoma or primary ovarian failure (POF)