4.5 Endocrine System

Overview

The endocrine system is a key player in maintaining homeostasis within the body using glands that secrete chemical signals called hormones into the bloodstream. These hormones are carried throughout the body, affecting various organs and tissues. Hormones play an important role in growth and development, as well as with metabolic activities.

Hormone Control

Hormone production and release are primarily controlled by negative feedback. In this way, the concentration of hormones in blood is maintained within a narrow range. For example, the pituitary signals the thyroid to release thyroid hormones. Increasing levels of these hormones in the blood then gives feedback to the hypothalamus and the pituitary to inhibit further signalling to the thyroid gland, so it will not continue to secrete thyroid hormone (Fowler et al., 2013).

The pathway is as follows:

1. The hypothalamus releases a specific chemical messenger called releasing factor.
2. The releasing factor stimulates the pituitary gland to release the next hormone, the tropic hormone, in the specific cascade.
3. The tropic hormone induces the release of the corresponding hormone from the endocrine gland.
4. This hormone will circulate in the bloodstream and interact with specific receptors to have an effect.

High levels of a specific hormone will feed back to the pituitary and hypothalamus to decrease the production of their corresponding secretions. This is callednegative feedback and overall decreases the production and release of the “end-game” hormone.

Note: Hormones travel in blood, so can encounter every organ in the body. However, they only have an effect on organs and structures that have receptors that recognize them.

Structures

Hypothalamus: A structure in the brain that produces and secretes many hormones

Pineal gland: A very small gland located in the brain whose functions are not entirely understood. Some of the specialized cells in the pineal gland are known to produce and secrete the hormone melatonin, which helps regulate circadian rhythms.

Pituitary gland: A small, bean-sized organ in the brain divided into two parts. The first part is the posterior pituitary, which does not produce hormones but stores and secretes hormones produced by the hypothalamus. The other part is the anterior pituitary, which does produce hormones.

Thyroid gland: A butterfly-shaped organ located on the ventral aspect of the trachea, just caudal to the larynx. It produces hormones that regulate the body’s metabolic rate and controls digestive, muscle, and heart functions, as well as brain development and bone maintenance.

Parathyroid glands: Tiny, round structures found on the surface of the thyroid gland. They produce and secrete parathyroid hormone, the major hormone involved in the regulation of blood calcium and phosphorus levels.

Adrenal glands: Found cranial to the kidneys, they have a rich blood supply and actually possess one of the highest rates of blood flow in the whole body. They produce hormones that help regulate metabolism, the immune system, blood pressure, the body’s response to stress, and other essential functions.

Pancreas: A long, slender organ located near the small intestine. Apart from its exocrine role of secreting digestive enzymes, it also has an endocrine function. The hormones produced by the pancreas, including insulin and glucagon, regulate blood glucose.

Ovaries: The female reproductive organs. The primary hormones produced by the ovaries are estrogens, which play a crucial role in the regulation of the estrus cycle, and progesterone, which is necessary to maintain pregnancy.

Testes: The male reproductive organs. The primary hormone produced by the testes is testosterone, which is a steroid hormone important in the development of the male reproductive system and the maturation of sperm cells.

Common Pathological Conditions of the Endocrine System

Addison’s disease: Also called hypoadrenocorticism; a condition caused by under-active adrenal glands – can be an emergency.

Cushing’s disease: Also called hyperadrenocorticism; a condition caused by the excessive production of cortisol by the adrenal cortex

Diabetes mellitus (DM): A chronic disease characterized by elevated blood sugar (hyperglycemia), which can be caused by under-production of insulin by the pancreas or if the receptors in the body are not properly responding to the insulin that is present

Hypercalcemia: Excessive calcium in the blood

Hyperkalemia: Excessive potassium in the blood

Hyperthyroidism: Excessive thyroid gland activity

Hypothyroidism: Deficient thyroid gland activity

Common Procedures

Most of the surgeries and procedures performed on the endocrine system involve the removal of a gland or an incision into a gland. Once an endocrine gland is surgically removed because of a tumour or enlargement, hormone replacement treatment is often required. Medication is used to synthetically replace the hormone produced by the gland and the function it regulates (Betts, et al., 2013).

ACTH stimulation test: Tests adrenal function; used to detect Cushing’s and Addison’s diseases

Adrenalectomy: The surgical removal of an adrenal gland

Baseline cortisol: A test of adrenal function; used to detect Addison’s disease

Baseline T4: A test of thyroid function; used to detect hyperthyroidism and hypothyroidism

Blood and urine glucose: A test used to detect diabetes mellitus

Chemical thyroidectomy: Also known as radioactive iodine therapy; a treatment for hyperthyroidism in cats. Radioactive iodine is used to decrease thyroid hormone production; it is generally seen as a permanent treatment and requires a special facility because of the radioactive components.

Dexamethasone suppression test: A test of adrenal function; used to detect Cushing’s disease

References

Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2013). Anatomy and physiology. OpenStax. https://openstax.org/details/books/anatomy-and-physiology, licensed under CC BY 4.0

Fowler, S., Roush R., Wise, J., Reeves, N., DeSaix, J., Kuehner, B., Leady, B., Boggs, L., Broverman, S., Byres, D., Marcus, B., Mhlanga, F., Mignone, M., Nash, E., Newton, M., Oliveras, D., Piperberg, J., Reisenauer, A., Rumfelt, L., Belk, M. … Zoubina, E. (2013). Concepts of biology. OpenStax. https://openstax.org/details/books/concepts-biology, licensed under CC BY 4.0

Image Credits
(images are listed in order of appearance)

Endocrine System by Kelly Robertson, NorQuest College. Used with permission.