Endocrine Glands And Hormones: Definition & Physiology

Endocrine Glands And Hormones: Function, Organs, Diseases

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:50 PM IST

Endocrine glands are those organs whose secretion is directly poured into the blood. These hormones regulate essential body functions like growth, metabolism, and reproduction. It helps to maintain a balance in the human body by coordinating various organs. In this article, the endocrine system, endocrine glands, and major endocrine glands and their hormones are discussed. Endocrine Glands and Hormones is a topic of the chapter Chemical Coordination and Integration chapter of Biology.

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The Endocrine System
What are Endocrine Glands?
Major Endocrine Glands and their Hormones

The Endocrine System

The endocrine system is an integrated system of ductless glands that function in the production and secretion of hormones into the general blood circulation, organizing and regulating activities within the body. That is, in a way, hormones can be likened to chemical messengers that move through the blood into target tissues or organs and control a wide range of physiological activities that commence from growth to metabolism, mood, and reproductive functions.

What are Endocrine Glands?

The endocrine glands in the human body are special organs that produce and secrete hormones directly into the bloodstream, thereby controlling such a wide array of physiological processes as those related to growth, metabolism, and homeostasis. Those include major glands such as the pituitary, thyroid, adrenals, and pancreas, which participate in the maintenance of internal balance.

Unlike exocrine glands, which release secretions through ducts to specific locations like sweat glands or salivary glands, the endocrine glands are ductless and require the circulatory system for transmission. This direct release into the circulatory system results in a diffuse action, systemic in nature, required for the coordination of activities of the body over long periods and responses to internal and external stimuli. The endocrine system plays a vital role in maintaining homeostasis, affecting growth and development, and regulating reproduction, therefore, keeping a person healthy and fit.

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Major Endocrine Glands and their Hormones

The major endocrine glands are:

Pituitary Gland

The pituitary gland is a pea-sized endocrine gland located at the base of the brain directly behind the socket of the eye and just below the hypothalamus. It is enclosed within a bony prominence of the sphenoid bone, the sella turcica.

Hormones Produced by Pituitary Gland

Growth Hormone (GH): Stimulation of growth and cell reproduction.
Prolactin: Milk production within the mammary glands.
Adrenocorticotropic Hormone (ACTH): Stimulation of the adrenal cortex to release cortisol.
Thyroid-Stimulating Hormone (TSH): Stimulation of the thyroid gland to produce thyroid hormones.
Follicle-stimulating hormone (FSH): It regulates the reproductive process. This includes activities such as maturation of ovarian follicles and spermatogenesis.
Luteinizing Hormone (LH): Ovulation stimulation and maintenance of sex hormone secretion.
AT or ADH: Water amount regulation in the body, water reabsorption in kidneys.
Oxytocin: Uterine contractions at birth, Milk letdown, lactation.

Functions and effects on the body

This is sometimes thought of as the "master gland" and does act in a manner somewhat similar to the pituitary gland. Such functions, through the release of hormones, actually do different facets of the body to control other endocrine glands. It has a very vital role in growth, metabolism, stress response, and reproductive processes.

Hypothalamus

Connection with the pituitary gland

The hypothalamus lies just above the pituitary gland and communicates with it through the hypothalamic-pituitary portal circulation, a system of blood vessels that enables the direct exchange of hormones between the two.

Hormones Produced by Hypothalamus

Thyrotropin Releasing Hormone (TRH): Stimulates TSH release from the pituitary.
Corticotropin Releasing Hormone (CRH): Stimulates ACTH release from the pituitary.
Gonadotropin-releasing hormone (GnRH): Controls the release of FSH and LH from the pituitary.
Growth Hormone Releasing Hormone: stimulates growth and development.
Somatostatin: Inhibits GH and TSH release.
Dopamine: Inhibits prolactin release.

Role in homeostasis and hormone regulation

Through its control over the activities of the pituitary, the hypothalamus indirectly regulates many of the body's functions, from overall growth and development to metabolic rate, responses to stress and reproductive activities.

Hypothalamus

Thyroid Gland

It is located anterior to the trachea in the neck. The thyroid gland has two lobes joined at an isthmus

Hormones Produced by Thyroid Gland

Thyroxine and Triiodothyronine, T4 and T3: This mediates the metabolic processes, producing energy, and growth.

Calcitonin: This reduces blood calcium through the inhibition of bone resorption and with an increased calcium deposition within the bones.

Functions and effects on metabolism, growth, and development

As mentioned, thyroid hormones regulate the metabolic rate, which affects growth. In addition, they are also necessary for maturation of nervous tissue.

Thyroid Hormones

Parathyroid Glands

The parathyroid gland has four small glands on the posterior surface of the thyroid gland.

Hormones Produced by Parathyroid Gland

Parathyroid Hormone (PTH): As a consequence of increased bone resorption, improved renal calcium reabsorption, and stimulation of intestinal calcium absorption, PTH brings up the level of calcium in the blood.

Functions and regulation of calcium levels

PTH controls the level of calcium required to keep the proper working of the muscles, transmission of nerve impulses, and correct bone health.

Parathyroid Hormone

Adrenal Glands

Two adrenal glands sitting at the top of each kidney are divided into two distinct parts—the cortex and the medulla.

Hormones Produced by Adrenal Glands

Cortisol: Controls the metabolism, and immune response, and enables the body's response to stress.

Aldosterone: Controls the balance of sodium-potassium. The product of this event affects blood pressure.

Adrenaline or Epinephrine and Noradrenaline or Norepinephrine: These are sometimes referred to as "fight-or-flight" hormones. During dangerous situations, they increase heart rate, blood pressure, and energy supply.

Functions and response to stress

The adrenal glands are mainly concerned with the management of stress reactions and controlling a wide array of metabolic and physiological activities.

Adrenal Glands

Pancreas

Dual role as an endocrine and exocrine gland.

The pancreas acts both as an endocrine gland concerning the production of hormones and as an exocrine gland concerning the secretion of digestive enzymes.

Hormones Produced by Pancreas

Insulin: It reduces blood sugar by increasing the uptake of glucose by cells.
Glucagon: It increases blood sugar by releasing glucose that is stored in the liver.
Somatostatin: This is against the release of both insulin and glucagon.

Functions and regulation of blood glucose levels

The pancreas controls the concentration of glucose in the blood through actions of insulin and glucagon. Hence this maintains homeostasis in energy

Hormones Released by Pancreas

Pineal Gland

Pineal Gland is a very small, pea-shaped gland, which rests in the epithalamus of the brain close to the middle of the brain.

Hormone Produced by Pineal Gland

Runs an individual's sleep-wake cycle and is also responsible for seasonal biological rhythms.

Functions and regulation of sleep-wake cycles

Melatonin appears to play a role in the production of the 'circadian rhythm' and sleep-wake cycles and, therefore, acts as the 'body clock.'

Gonads (Testes and Ovaries)

Testes are in the scrotum whereas ovaries are in the pelvic cavity.

Hormones produced:

Testosterone: It controls male reproductive functions as well as secondary sexual characteristics.
Estrogen and Progesterone: They regulate female reproductive activity, the menstrual cycle, and the manifestation of secondary sexual characteristics.

Functions and roles in reproduction and secondary sexual characteristics

Gonads play a crucial role in reproduction and in expressing secondary sexual traits and, therefore impact the physical characteristics and behaviours defining sexual maturation.

Also Read:

Mechanism of Hormone Action	Hormone Receptors
Adrenal Cortex	Adrenal Medulla
Difference between Thyroid and Parathyroid	Adrenal Insufficiency

Frequently Asked Questions (FAQs)

1. What is the endocrine system and its function?

It is referred to as a couple of organs called glands that are in charge of producing and secreting hormones to regulate a few activities within the body, such as metabolism, growth, and reproduction.

2. What are the major endocrine glands in the human body?

Major endocrine glands include the pituitary, hypothalamus, thyroid, parathyroid, adrenals, pancreas, pineal, and gonads.

3. How do hormones affect the body?

Hormones are chemical messengers in the blood that act on tissues and organs to control metabolism, growth, mood, and reproductive functions.

4. What are common disorders of the endocrine system?

Common disorders of this kind include diabetes mellitus, hypothyroidism, hyperthyroidism, Cushing's syndrome, Addison's disease, and growth hormone disorders like gigantism and dwarfism.

5. How are endocrine disorders diagnosed and treated?

Diagnosis of endocrine disorders is made by blood tests, imaging techniques and physical examinations; treatment may be in the form of hormone replacement therapy, drugs and lifestyle changes; at times, surgical intervention may also be required.

6. What is the function of the thyroid gland?

The thyroid gland produces thyroxine and triiodothyronine which are involved in the regulation of the metabolic rate, growth and development and calcitonin, which is responsible for calcium homeostasis.

7. How does the pancreas function as an endocrine gland?

The pancreas secretes hormones like insulin and glucagon and maintains the blood sugar at a normal level.

8. What is the hypothalamus-pituitary axis and why is it important?

The hypothalamus-pituitary axis is a complex interaction between the hypothalamus and pituitary gland in the brain. It's crucial for regulating many bodily functions as the hypothalamus controls the pituitary, which in turn influences other endocrine glands. This axis plays a central role in maintaining homeostasis and coordinating responses to stress, growth, reproduction, and metabolism.

9. How does the thyroid gland regulate metabolism?

The thyroid gland produces hormones (mainly thyroxine and triiodothyronine) that regulate metabolism by controlling the rate at which cells convert nutrients into energy. These hormones increase the basal metabolic rate, affecting heart rate, body temperature, and energy consumption. Thyroid hormones also play crucial roles in growth and development.

10. How do adrenal glands respond to stress?

Adrenal glands respond to stress by releasing hormones like cortisol and adrenaline (epinephrine). These hormones trigger the "fight or flight" response, increasing heart rate, blood pressure, and blood sugar levels while suppressing non-essential functions like digestion. This prepares the body to deal with perceived threats or challenges.

11. How does growth hormone affect the body throughout life?

Growth hormone, produced by the pituitary gland, stimulates growth and cell reproduction in children and adolescents. In adults, it continues to regulate metabolism, bone density, and muscle mass. It also plays a role in fat distribution and cardiovascular health. Growth hormone levels naturally decline with age, contributing to age-related changes in body composition.

12. What is the difference between type 1 and type 2 diabetes?

Type 1 diabetes is an autoimmune condition where the body's immune system destroys insulin-producing cells in the pancreas, leading to little or no insulin production. Type 2 diabetes, more common, occurs when cells become resistant to insulin or the pancreas doesn't produce enough insulin. Both types result in high blood sugar levels but have different causes and treatments.

13. What is the endocrine system and how does it differ from the nervous system?

The endocrine system is a network of glands that produce and secrete hormones directly into the bloodstream. Unlike the nervous system, which uses electrical impulses for rapid communication, the endocrine system uses chemical messengers (hormones) for slower, longer-lasting effects throughout the body. This system regulates various bodily functions such as metabolism, growth, development, reproduction, and mood.

14. How does the endocrine system maintain homeostasis?

The endocrine system maintains homeostasis by constantly monitoring and adjusting various bodily functions. It uses feedback mechanisms to detect changes in the internal environment and releases appropriate hormones to counteract these changes. For example, it regulates blood sugar levels, body temperature, blood pressure, and fluid balance, ensuring that the body's internal conditions remain stable despite external changes.

15. How does negative feedback work in the endocrine system?

Negative feedback is a regulatory mechanism where the output of a system inhibits further production of that output. In the endocrine system, when hormone levels rise, they signal the gland to stop or slow down hormone production. This helps maintain hormone levels within a normal range, ensuring body functions remain balanced.

16. How do endocrine disruptors affect hormone function?

Endocrine disruptors are chemicals that interfere with the normal functioning of hormones. They can mimic hormones, block hormone receptors, or alter hormone production and breakdown. This disruption can lead to various health issues, including developmental problems, reproductive disorders, and certain cancers. Common sources of endocrine disruptors include some plastics, pesticides, and industrial chemicals.

17. How does the endocrine system change with aging?

As we age, the endocrine system undergoes several changes. Hormone production often decreases, and target tissues may become less sensitive to hormones. Common age-related changes include decreased growth hormone and sex hormone production, reduced thyroid function, and alterations in insulin sensitivity. The hypothalamus and pituitary gland may become less responsive to feedback mechanisms. These changes can contribute to various age-related conditions, including osteoporosis, metabolic disorders, and changes in body composition.

18. What is the role of prolactin beyond lactation?

While prolactin is best known for its role in milk production, it has numerous other functions. It influences reproductive behavior, immune system regulation, and metabolism. Prolactin affects osmoregulation (water and salt balance), particularly in fish and amphibians. It also plays a role in paternal behavior in some species. In humans, prolactin may influence mood, sexual satisfaction, and immune function, though its full range of effects is still being studied.

19. What is the role of cortisol in the body beyond stress response?

While cortisol is often known as the "stress hormone," it has numerous functions beyond stress response. It helps regulate metabolism by influencing blood sugar levels, protein breakdown, and fat metabolism. Cortisol plays a role in the immune system by reducing inflammation. It affects bone formation, influences blood pressure regulation, and contributes to the sleep-wake cycle. Cortisol also impacts memory formation and retrieval, particularly in stressful situations.

20. How do thyroid hormones influence fetal development?

Thyroid hormones are crucial for fetal development, particularly for brain and nervous system development. They influence the growth and differentiation of neurons, myelination of nerve fibers, and the formation of synapses. Thyroid hormones also affect skeletal development, lung maturation, and overall growth. Maternal thyroid function is especially important in early pregnancy before the fetal thyroid gland becomes functional. Thyroid hormone deficiency during pregnancy can lead to developmental delays and cognitive impairment in the child.

21. What is the function of glucagon in glucose regulation?

Glucagon, produced by the pancreas, works opposite to insulin in regulating blood glucose levels. When blood sugar is low, glucagon stimulates the liver to break down stored glycogen into glucose and release it into the bloodstream. It also promotes gluconeogenesis (the production of new glucose) and the breakdown of fats for energy. This hormone helps maintain blood glucose levels between meals and during fasting periods, preventing hypoglycemia.

22. How do growth factors differ from other hormones?

Growth factors are a special class of hormones that stimulate cell growth, proliferation, and differentiation. Unlike many classical hormones that act systemically, growth factors often work locally, affecting cells near where they are produced. They can act in an autocrine (on the cell that produced them), paracrine (on nearby cells), or endocrine (via bloodstream) manner. Examples include insulin-like growth factors (IGFs), epidermal growth factor (EGF), and fibroblast growth factors (FGFs). Growth factors play crucial roles in development, wound healing, and tissue maintenance.

23. What is the difference between peptide and steroid hormones?

Peptide hormones are made of amino acids and are water-soluble. They bind to receptors on the cell surface and cannot enter cells directly. Steroid hormones, derived from cholesterol, are fat-soluble and can pass through cell membranes to bind with intracellular receptors. This difference affects how quickly they act and how long their effects last.

24. What is the role of ghrelin in hunger and metabolism?

Ghrelin, often called the "hunger hormone," is primarily produced in the stomach. It stimulates appetite and promotes fat storage. Ghrelin levels typically rise before meals and fall after eating. It also plays a role in regulating energy homeostasis, influencing metabolism and body weight. Ghrelin has additional functions, including effects on sleep, reward-seeking behavior, and memory formation.

25. What is the role of oxytocin in social bonding and childbirth?

Oxytocin, often called the "love hormone," plays a crucial role in social bonding and childbirth. During childbirth, it stimulates uterine contractions and milk ejection for breastfeeding. In social contexts, oxytocin promotes trust, empathy, and attachment between individuals. It's released during positive social interactions, physical touch, and sexual activity, contributing to emotional bonding and relationship formation.

26. What is the function of aldosterone in the body?

Aldosterone, produced by the adrenal glands, plays a crucial role in regulating blood pressure and electrolyte balance. It acts on the kidneys to increase sodium reabsorption and potassium excretion. This sodium retention leads to water retention, increasing blood volume and pressure. Aldosterone is part of the renin-angiotensin-aldosterone system, which is important for maintaining proper blood pressure and fluid balance.

27. How does the thymus gland contribute to immune function?

The thymus gland plays a critical role in the development of the immune system, particularly in early life. It produces and educates T-lymphocytes (T-cells), which are crucial for cell-mediated immunity. The thymus teaches T-cells to recognize and respond to foreign substances while not attacking the body's own tissues. Although the thymus shrinks with age, its early role in immune system development is vital for lifelong immunity.

28. How do hormones work at the cellular level?

Hormones work by binding to specific receptors on target cells. This binding triggers a cascade of intracellular events, leading to changes in cell function, gene expression, or metabolism. Some hormones bind to receptors on the cell surface, while others (like steroid hormones) can pass through the cell membrane and bind to receptors inside the cell.

29. What is the role of insulin in glucose regulation?

Insulin, produced by the pancreas, regulates blood glucose levels by facilitating the uptake of glucose into cells, especially in muscle and fat tissue. It also promotes the storage of excess glucose as glycogen in the liver and muscles. When insulin function is impaired, it can lead to diabetes mellitus, a condition characterized by high blood sugar levels.

30. What is the role of melatonin in sleep-wake cycles?

Melatonin, produced by the pineal gland, regulates the body's circadian rhythm or sleep-wake cycle. Its production increases in darkness and decreases in light, signaling to the body when it's time to sleep. Melatonin helps control the timing of our sleep patterns and may influence sleep quality, making it crucial for maintaining a healthy sleep schedule.

31. How do sex hormones influence secondary sexual characteristics?

Sex hormones (estrogen, testosterone, and progesterone) influence the development of secondary sexual characteristics during puberty. In males, testosterone promotes facial hair growth, deepening of the voice, and muscle development. In females, estrogen and progesterone contribute to breast development, widening of hips, and regulation of the menstrual cycle. These hormones continue to play important roles in reproduction and overall health throughout adulthood.

32. What is the function of parathyroid hormone?

Parathyroid hormone (PTH), produced by the parathyroid glands, regulates calcium levels in the blood. It increases blood calcium by promoting calcium absorption from the intestines, reabsorption in the kidneys, and release from bones. PTH also decreases phosphate levels in the blood. This hormone is crucial for maintaining proper bone density, muscle function, and nerve signaling.

33. What is the difference between endocrine and exocrine glands?

Endocrine glands secrete hormones directly into the bloodstream, affecting distant target organs. Exocrine glands, on the other hand, release their products through ducts onto external or internal body surfaces. Examples of exocrine glands include sweat glands and salivary glands.

34. Why are hormones often referred to as "chemical messengers"?

Hormones are called chemical messengers because they carry information from one part of the body to another through the bloodstream. They act as signaling molecules, instructing target cells to perform specific functions or make changes in their activity, thus coordinating various bodily processes.

35. How do hormones influence mood and behavior?

Hormones significantly influence mood and behavior by affecting neurotransmitter levels and neural pathways in the brain. For example, serotonin, often influenced by estrogen, affects mood and anxiety. Cortisol, the stress hormone, can impact cognitive function and emotional regulation. Testosterone can influence aggression and risk-taking behavior. Hormonal fluctuations, such as those during the menstrual cycle or puberty, can lead to mood swings and behavioral changes.

36. How do hormones influence bone growth and density?

Several hormones influence bone growth and density. Growth hormone and insulin-like growth factor 1 (IGF-1) promote bone growth and development. Sex hormones (estrogen and testosterone) help maintain bone density by regulating bone remodeling. Parathyroid hormone and calcitonin work together to regulate calcium levels, which is crucial for bone health. Thyroid hormones also play a role in bone metabolism. Imbalances in these hormones can lead to conditions like osteoporosis or abnormal bone growth.

37. What is the relationship between leptin and appetite regulation?

Leptin, produced by fat cells, is often called the "satiety hormone." It helps regulate energy balance by inhibiting hunger. Leptin levels increase as fat storage increases, signaling the brain to reduce appetite and increase energy expenditure. However, in obesity, individuals often develop leptin resistance, where the brain becomes less responsive to leptin's signals, leading to continued appetite and weight gain despite high leptin levels.

38. How does the pineal gland respond to light and dark cycles?

The pineal gland responds to light and dark cycles through its connection to the eyes via the suprachiasmatic nucleus in the brain. In darkness, it produces melatonin, which promotes sleep. Light exposure suppresses melatonin production. This mechanism helps regulate the body's circadian rhythms, synchronizing physiological processes with the external day-night cycle. The pineal gland's response to light is crucial for maintaining proper sleep-wake patterns and other daily biological rhythms.

39. What is the function of antidiuretic hormone (ADH)?

Antidiuretic hormone (ADH), also known as vasopressin, is produced by the hypothalamus and released by the posterior pituitary. Its primary function is to regulate water balance in the body. ADH increases water reabsorption in the kidneys, reducing urine output and helping to maintain blood volume and pressure. It also causes blood vessel constriction, further supporting blood pressure regulation. ADH deficiency can lead to diabetes insipidus, characterized by excessive thirst and urination.

40. What is the function of erythropoietin (EPO)?

Erythropoietin (EPO) is a hormone primarily produced by the kidneys in response to low oxygen levels in the blood. Its main function is to stimulate the production of red blood cells (erythropoiesis) in the bone marrow. By increasing the number of red blood cells, EPO helps improve oxygen delivery to tissues throughout the body. This hormone is crucial for adapting to high altitudes and is also used medically to treat certain types of anemia.

41. How do hormones regulate the menstrual cycle?

The menstrual cycle is regulated by a complex interplay of hormones. Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates the pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones stimulate the ovaries to produce estrogen and progesterone. Estrogen promotes the growth of the uterine lining, while progesterone prepares it for potential pregnancy. If pregnancy doesn't occur, hormone levels drop, triggering menstruation. This cyclical process is controlled by positive and negative feedback mechanisms.

42. What is the role of calcitonin in calcium homeostasis?

Calcitonin, produced by the thyroid gland, works alongside parathyroid hormone to regulate calcium levels in the blood. While parathyroid hormone increases blood calcium, calcitonin decreases it. Calcitonin inhibits bone resorption by osteoclasts, reducing the release of calcium from bones into the blood. It also promotes calcium excretion by the kidneys and reduces calcium absorption in the intestines. This hormone is particularly important in protecting against excessive bone loss during periods of high calcium demand, such as pregnancy and lactation.

43. How does the endocrine system interact with the immune system?

The endocrine and immune systems have a complex, bidirectional relationship. Many hormones, including cortisol, growth hormone, and thyroid hormones, influence immune function. For example, cortisol generally suppresses immune responses, while growth hormone and prolactin can enhance them. Conversely, immune cells can produce hormones and influence endocrine gland function. Cytokines, produced by immune cells, can affect hormone production and action. This interaction is crucial for maintaining health and responding to stress and disease.

44. What is the function of inhibin and its role in reproduction?

Inhibin is a hormone produced by the gonads (ovaries in females, testes in males). Its primary function is to inhibit the production and release of follicle-stimulating hormone (FSH) from the pituitary gland. In females, inhibin helps regulate the menstrual