An introduction to the endocrine system
I’ll let you in on a secret.
I hated the endocrine system when I was in college. It was the most confusing system we had to learn, and now it is my absolute favourite.
Endocrine patients provide us with so many great nursing opportunities. These patients have so many complex needs - from diagnosis, stabilisation of emergency patients and inpatient care, right up to the end of their life, the veterinary nurse is heavily involved.
Today we’re going right back to basics and starting with an overview of the endocrine system itself. I’ll be talking you through the major glands in the system, how they work, the hormones they produce, and their target organs.
This post kicks off a new endocrine series here on the blog, as well as over on instagram and facebook - so stay tuned as we demystify the most complicated system in the body!
The Pituitary Gland
The pituitary gland is a major site of hormone storage, production and secretion. It sits just at the base of the brain and can be divided into two portions - the anterior and posterior pituitary portions. The pituitary gland is connected to the hypothalamus, which stimulates it to release various hormones.
There are several major hormones produced by the pituitary gland:
Thyroid Stimulating Hormone
TSH is released by the anterior pituitary and works on the thyroid glands, where it stimulates them to secret thyroid hormones (T3 and T4). The release of TSH is stimulated by the hypothalamus, which releases TRH (thyrotropin-releasing hormone) to stimulate the pituitary gland. When thyroid hormone levels are within normal limits, a negative feedback system is activated, which tells the hypothalamus to stop stimulating the pituitary gland, in turn, meaning the thyroid glands do not receive any TSH.
Adrenocorticotropic Hormone
ACTH works on the cortex of the adrenal glands, stimulating them to release cortisol. It is released by the anterior pituitary. Like with the thyroid gland, ACTH release is regulated by the hypothalamus and a negative feedback system. CRH (corticotropin-releasing hormone) is released by the hypothalamus, causing the pituitary to release ACTH. When cortisol levels are normal, CRH and ACTH release cease.
Somatotropin
Somatotropin, or growth hormone, is released by the anterior pituitary gland. Growth hormone has many target organs, but major organs include the liver and fat tissue. It is responsible, as the name suggests, for maintaining normal growth!
Antidiuretic Hormone
ADH stimulates the kidney to reabsorb water and concentrate the urine. It is produced by the posterior pituitary.
In addition to these major hormones, the pituitary gland is responsible for producing several reproductive hormones, such as oxytocin, prolactin, luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
The Thyroid Glands
The thyroid glands are paired glands located in the ventral neck, either side of the trachea, at the level of the 5-6th tracheal rings.
The glands consist of thyroid epithelial cells and parafollicular cells. Epithelial cells synthesize the two thyroid hormones thyroxine (T4) and triiodothyronine (T3), from dietary iodine. The main role of the thyroid hormones is to increase the body’s basal metabolic rate.
Parafollicular cells secrete calcitonin, which counteracts the effects of parathyroid hormone, reducing circulating calcium levels.
The Adrenal Glands
The adrenal glands are paired glands sitting in close proximity to the kidneys (ad-renal = added to the kidneys!).
They are made up of an outer cortex and inner medulla.
The cortex is responsible for the synthesis and release of several steroids - cortisol, aldosterone and androgens.
Adrenal Hormones
Cortisol is a kind of steroid called a glucocorticoid. It plays a key role in a number of normal body processes, including the stress response, maintenance of a healthy gastrointestinal tract, and much more.
Aldosterone is a mineralocorticoid. The main function of aldosterone is to maintain fluid and electrolyte balance. Aldosterone acts in the kidney, where it encourages the excretion of potassium and retention of sodium (and, along with that, retention of fluid).
The adrenal medulla is responsible for the release of our ‘fight or flight’ hormones - adrenaline, noradrenaline and dopamine.
Release and Maintenance
The release of cortisol from the adrenals is triggered by ACTH release from the pituitary gland. The whole system is regulated by a negative feedback mechanism, so if there are sufficient circulating cortisol levels the pituitary ‘shuts off’ ACTH release until levels fall.
Aldosterone release is regulated by the Renin - Angiotensin - Aldosterone System (RAAS).
The release of our catecholamines from the adrenal medulla is mediated by the sympathetic nervous system.
The Pancreas
The pancreas is a bilobed organ which sits in the cranial abdomen, between the stomach and the duodenum.
The pancreas is an organ of both exocrine and endocrine functions – the pancreatic acinar cells are responsible for the formation and secretion of digestive enzymes, and the endocrine cells within the Islets of Langerhans are responsible for glucose homeostasis. This is mainly achieved through the hormones glucagon and insulin.
Glucagon is secreted from alpha cells within the Islets, and increases blood glucose levels by converting stored glycogen in the liver into glucose.
Insulin is released from beta cells in response to elevated blood glucose levels. This hormone reduces blood glucose by stimulating fat, liver and muscle tissue to store glucose, reducing circulating glucose levels.
There are two other hormones/peptides secreted by cells within the pancreatic Islets - pancreatic polypeptide, and somatostatin.
Somatostatin is secreted from delta cells, and inhibits growth hormone, whereas pancreatic polypeptide is secreted from F (or PP) cells. This is responsible for maintaining pancreatic function, and plays a role in storing glycogen within the liver.
The Parathyroid Glands
The parathyroids are four small glands located next to the thyroid glands. Their main role is in regulating blood calcium levels within the body.
Parathyroid hormone (PTH) is secreted by the parathyroid glands in response to hypocalcaemia.
Parathyroid hormone converts vitamin D to its biologically active form, vitamin D3 (calcitriol). This increases calcium levels through three processes:
CIncreasing absorption of calcium through the GI tract
Conserving calcium in the kidneys
Releasing calcium from bony stores.
When calcium levels are normal, the negative feedback system is activated and PTH release is stopped.
So that’s an overview of the main endocrine glands in the body, and how they work! This is really important for us to know as nurses, as endocrine diseases are so common, and each condition results in a different disruption in these normal systems. This helps us understand the signs we see, and why we see them - which in turn helps us plan better nursing care.
Want to learn more about nursing the endocrine patient? Grab yourself one of our VIMN endocrine guides, your go-to in practice reference to nursing patients with endocrine diseases. Pick one up here!
References:
Merrill, L. 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.
