The Endocrine System—A Brief Overview

The endocrine system is involved in regulating different functions and communication between organs in the body through the release of hormones, which are the chemical messengers of the body. Hormones are secreted into the bloodstream by ductless endocrine glands and transported to their target cells, where they bind to receptors inside or on the surface of the cell. Hormones contribute to metabolism, growth and development, sleep, reproduction, blood pressure, and emotions.1–4

There are three classes of hormones—steroids, amino acids, and polypeptides. 

Steroids are derived from cholesterol and produced by the gonads and adrenal cortex. They can enter target cells and bind to receptors in the cytoplasm or nucleus, where they then bind to parts of deoxyribonucleic acid (DNA) to regulate the activity of hormone-responsive genes.

Amino acid derivatives are produced by the thyroid and adrenal medulla. They can enter target cells and bind to receptors associated with specific regions of DNA, thereby affecting the activity of certain genes.

Polypeptides are made of amino acid chains that form part of or the whole of a protein molecule and are found in the hypothalamus, pituitary gland, and pancreas. They cannot enter cells, instead binding to receptors on the cell surface, which causes changes to the cell’s function.4

 Most hormones are controlled by a negative feedback mechanism in order to maintain stability and a state of equilibrium in the body, or homeostasis. This means that when hormone levels reach a certain point, the hypothalamus and/or pituitary glands, which are responsible for regulating hormone production and secretion, cease hormone secretion. When hormone levels fall below a certain point, hormone production and secretion resume.3,4

Glands

The endocrine system is composed of glands located throughout the body. Each gland produces hormones with different capabilities.

Hypothalamus. Located in the brain, the hypothalamus serves as the link between the endocrine and nervous systems, using information from the nervous system to determine whether other endocrine glands, particularly the pituitary gland, should produce hormones. It is responsible for many biological processes, including hunger and thirst, emotions, sleep patterns, body temperature, sexual function, blood pressure, and heart rate.3,4  

Hormones produced by the hypothalamus include vasopressin, or anti-diuretic hormone, which regulates blood pressure, electrolyte levels; water retention in the kidneys; oxytocin, which stimulates uterine contractions and milk ejection;2,4–6 dopamine, which inhibits secretion of prolactin;5 somatostatin, which inhibits the release of growth hormone (GH);4,5 GH-releasing hormone, which stimulates the release of GH; corticotropin-releasing hormone, which regulates adrenocorticotrophic hormone (ACTH) release; gonadotropin-releasing hormone, which stimulates luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release and contributes to sexual and reproductive processes; and thyrotropin-releasing hormone, which regulates thyroid stimulating hormone (TSH) secretion.4–6

Pituitary gland. The pituitary gland is located at the base of the brain and is composed of an anterior and a posterior lobe. The anterior lobe makes hormones that control production of certain other hormones (e.g., ACTH, LH, FSH, TSH) in other glands, as well as impact specific organs.3,4 ACTH stimulates production of corticosteroid hormones in the adrenal cortex, LH and FSH regulate sex hormone production in the gonads, and TSH stimulates thyroid hormone production in the thyroid. Prolactin and GH act directly on target organs. GH plays a pivotal role in growth and metabolism, and prolactin initiates milk production.2,4–6

The posterior pituitary lobe does not produce hormones, instead storing and secreting oxytocin and vasopressin.4 

Pineal gland. The pineal gland regulates sleep patterns and is the only significant producer of melatonin in the body.5 

Thyroid. Located in the neck, the thyroid produces thyroxine (T4) and triiodothyronine (T3), collectively referred to as thyroid hormone.4 Thyroid hormone controls metabolic activity and function in almost every organ. It contributes to the development of the central nervous system, and it affects overall growth and maturation as well.4–6

Parathyroid glands. There are four parathyroid glands located behind the thyroid that produce parathyroid hormone (PTH). PTH regulates blood calcium levels and maintains phosphate homeostasis.4,5  

Adrenal glands. The adrenal glands are located on top of the kidneys and are composed of an adrenal cortex, which is the outer layer, and an adrenal medulla, which is  the inner layer. 

The adrenal cortex produces corticosteroids, such as cortisol and aldosterone. Cortisol primarily increases blood glucose levels, which, in turn, increases insulin resistance. Cortisol can also decrease inflammation and immune responses and protect against the effects of stress.4–6 Aldosterone maintains water and electrolyte balance4–6 and helps regulate blood pressure.6 

The adrenal medulla produces epinephrine (adrenaline) and norepinephrine (noradrenaline), which are often released simultaneously in response to stress or strong emotions (e.g., fear, anger).4,5   

Pancreas. Located behind the stomach, the pancreas is the largest endocrine gland, and it serves to regulate blood glucose levels.3,4 The pancreas produces insulin and glucagon, which increase and decrease blood glucose, respectively. Additionally, insulin promotes fat and protein metabolism.4–6

Gonads. The gonads (i.e., ovaries and testes) are responsible for the production of sex hormones, which are needed for the development and function of reproductive organs and secondary sex characteristics and production of germ cells (i.e., ova and spermatozoa).4 The ovaries primarily produce estrogens, which regulate the menstrual cycle, maintain libido, and protect bone health; progesterone, which prepares the uterine lining for pregnancy and the mammary glands for milk production; and testosterone, which affects muscles, reproductive tissue, growth, and psychological behavior.4–6 During menopause, the ovaries stop producing estrogens, causing symptoms such as hot flashes, heart palpitations, and brittle bones.4 The testes primarily produce testosterone, which increases muscle and bone mass, maintains libido, regulates sperm production, and regulates the production of red blood cells and hematocrit levels.4–6 

Nutrition and the Endocrine System

Proper nutrition is important for the functioning of the endocrine system, as nutritional impairments can disrupt the endocrine system and potentially contribute to endocrine disorders. For instance, anorexia nervosa is associated with hypothalamic amenorrhea, or a disruption of the menstrual cycle due to disruptions in the function gonadotropin-releasing hormone and low levels of LH and estrogens, and increased levels of cortisol have been observed in individuals with anorexia nervosa, bulimia, or eating disorders not otherwise specified.7 

Adequate protein consumption is crucial for the endocrine system to function properly because they contain amino acids, the building blocks for polypeptides. Protein intake can increase satiety through the stimulation of certain hormones as well,8 and increased protein consumption (20–30% total energy intake) has been linked to improvements in Type 2 diabetes.9 Chicken breasts, legumes, eggs, fish, and reduced-fat dairy products are excellent providers of protein.8,9

Key to the functioning of the pancreas are carbohydrates, which break down into the glucose that informs the production of glucagon and insulin. People with diabetes on flexible insulin dosing therapies must carefully monitor their carbohydrate intake to determine the amount of insulin they must take. Those on a fixed insulin regimen should consume a consistent amount of carbohydrates at each meal and snack. Carbohydrate sources for people with diabetes should also be high in fiber, which helps regular blood sugar levels.9,10 Healthy sources of carbohydrates, for those with or without diabetes, include whole grain pasta and bread, brown rice, legumes, steel-cut or old-fashioned oats, and whole fruits and vegetables.9–11  

Thyroid disorders affect about 20 million Americans and are five times more likely to appear in women than men.3 Iodine deficiency is associated with goiter (i.e., enlargement of the thyroid) and hypothyroidism (i.e., when the body makes too little thyroid hormone), and iodine deficiency during pregnancy can lead to intellectual disabilities and stunted growth in the child. Dietary sources of iodine include iodized salt, saltwater fish and shellfish, eggs, seaweed, dairy products, soy milk, and soy sauce.12,13 Individuals with hypothyroidism or iodine deficiency should be cautious of overconsuming cruciferous vegetables (e.g., broccoli, Brussels sprouts), as they contain goitrogens that can prevent iodine absorption.13 However, individuals with autoimmune thyroid disease, chronic iodine deficiency, and hyperthyroidism must be careful of excessive iodine consumption, which  can cause or worsen hypothyroidism or hyperthyroidism.12,13

Vitamin A also plays a role in managing homeostasis in the thyroid. Vitamin A deficiency has been associated with iodine deficiency, and studies have shown that vitamin A supplementation in patients with both deficiencies has a beneficial effect on the thyroid.14 Sources of vitamin A include leafy green vegetables, tomatoes, red bell pepper, milk, eggs, and cantaloupe.15 Studies have shown that vitamin D deficiency is associated with an increased risk of developing hypothyroidism and other thyroid diseases, though further research is needed to better understand this association.16 The micronutrient selenium is also essential for proper thyroid functioning. Selenium supplementation is correlated with improvements in autoimmune thyroid disease.17,18 Foods high in selenium include Brazil nuts, organ meats, seafood, bread, and grains.18,19

Editor’s note: Consult with a qualified healthcare professional or dietitian/nutritionist to determine a diet that best meets your nutritional needs.

Sources

  1. National Cancer Institute. SEER training modules. Introduction to the endocrine system. https://training.seer.cancer.gov/anatomy/endocrine/. Accessed 15 Nov 2022.
  2. Young WF Jr. Overview of the endocrine system. Merck Manuals Professional Edition. Updated Sep 2022. https://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/principles-of-endocrinology/overview-of-the-endocrine-system. Accessed 15 Nov 2022.
  3. Cleveland Clinic. Endocrine system. Reviewed 12 May 2020. https://my.clevelandclinic.org/health/articles/21201-endocrine-system. Accessed 15 Nov 2022.
  4. Hiller-Sturmhöfel S, Bartke A. The endocrine system: an overview. Alcohol Health Res World. 1998;22(3):153–164. 
  5. Campbell M, Jialal I. Physiology, endocrine hormones. Updated 26 Sep 2022. In: StatPearls [Internet]. StatPearls Publishing; 2022 Jan.
  6. Johns Hopkins Medicine. Hormones and the endocrine system. https://www.hopkinsmedicine.org/health/conditions-and-diseases/hormones-and-the-endocrine-system. Accessed 15 Nov 2022.
  7. Warren MP. Endocrine manifestations of eating disorders. J Clin Endocrinol Metab. 2011;96(2):333–343.
  8. Lang A. 10 natural ways to balance your hormones. Healthline. Updated 31 Jan 2022. https://www.healthline.com/nutrition/balance-hormones#TOC_TITLE_HDR_2. Accessed 16 Nov 2022.
  9. Gray A, Threlkeld RJ. Nutritional recommendations for individuals with diabetes. Updated 13 Oct 2019. In: Feingold KR, Anawalt B, Boyce A, et al, eds. Endotext [Internet]. MDText.com, Inc.; 2000.
  10. Diabetes Institute University of Florida. Type 1 diabetes nutrition. https://diabetes.ufl.edu/outreach/resources/nutrition/type-1-diabetes/. Accessed 16 Nov 2022.
  11. Harvard TH Chan School of Public Health. Carbohydrates. https://www.hsph.harvard.edu/nutritionsource/carbohydrates/. Accessed 16 Nov 2022.
  12. American Thyroid Association. Iodine deficiency. https://www.thyroid.org/iodine-deficiency/. Accessed 16 Nov 2022.
  13. Harvard TH Chan School of Public Health. Iodine. https://www.hsph.harvard.edu/nutritionsource/iodine/. Accessed 16 Nov 2022.
  14. Capriello S, Stramazzo I, Bagaglini MF, et al. The relationship between thyroid disorders and vitamin A.: a narrative minireview. Front Endocrinol (Lausanne). 2022;13:968215. 
  15. Harvard TH Chan School of Public Health. Vitamin A. https://www.hsph.harvard.edu/nutritionsource/vitamin-a/. Accessed 16 Nov 2022.
  16. Ashok T, Palyam V, Azam A T, et al. Relationship between vitamin D and thyroid: an enigma. Cureus. 2022;14(1):e21069.
  17. Ventura M, Melo M, Carrilho F. Selenium and thyroid disease: from pathophysiology to treatment. Int J Endocrinol. 2017;2017:1297658. 
  18. Gorini F, Sabatino L, Pingitore A, Vassalle C. Selenium: an element of life essential for thyroid function. Molecules. 2021;26(23):7084. 
  19. National Institutes of Health Office of Dietary Supplements. Selenium. Updated 26 Mar 2021. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/#h3. Accessed 16 Nov 2022.   

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