About this course:
The purpose of this module is to provide the user with an overview of the anatomy and physiology of the endocrine system and its hormones.
Anatomy and Physiology of the Endocrine System
The purpose of this module is to provide the user with an overview of the anatomy and physiology of the endocrine system and its hormones.
Upon completion of this CE activity, the learner will be able to do the following:
- describe the anatomical and physiological features of the endocrine system
- recognize the various hormones produced and regulated by the endocrine system
- identify the feedback mechanism and basic causes of dysfunction of the endocrine system
The endocrine system is a complex system that works in tandem with the nervous system to maintain the delicate balance of homeostasis. An understanding of the anatomy and physiology of the endocrine system is necessary to determine the impact of any alterations in function leading to pathological disorders/conditions. Disorders of the endocrine system contribute significantly to healthcare expenses each year. Aside from the monetary costs, the loss of work, impaired quality of life, and ongoing personal disparities caused by endocrine disorders pose a significant burden to patients and their families. It’s estimated that the prevalence of endocrine disorders is over 5% of the adult population in the US for each of the major diseases, including diabetes mellitus (DM), obesity, metabolic syndrome, osteoporosis, erectile dysfunction, dyslipidemia, and thyroiditis. Diabetes mellitus is a commonly diagnosed endocrine disorder in the US that affects over 10% of the population. According to the Centers for Disease Control and Prevention (CDC, 2023), DM occurs most often among patients of Black, Hispanic American, and Latino American ethnicity. Thyroid disorders, osteoporosis, and polycystic ovary syndrome (PCOS) are the most common endocrine disorders affecting female patients, while erectile dysfunction and osteopenia are the most common endocrine disorders in male patients (CDC, 2023).
If the body does not respond to hormones appropriately, a variety of endocrine diseases or disorders can occur, including but not limited to:
- hypothalamus/pituitary disorders
- growth hormone (GH) deficiency
- diabetes insipidus (DI)
- central hypothyroidism
- secondary hypogonadism, including secondary amenorrhea, functional hypothalamic amenorrhea (FHA), or delayed puberty/primary amenorrhea
- acromegaly and gigantism
- syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- thyroid disorders
- primary hypothyroidism
- primary hyperthyroidism
- adrenal dysfunction
- adrenal insufficiency
- hypercortisolism (Cushing’s syndrome)
- parathyroid dysfunction
- ovarian dysfunction
- polycystic ovary syndrome (PCOS)
- primary ovarian insufficiency (POI)
- testicular dysfunction
- primary hypogonadism
- islet cells of the pancreas
- Diabetes Mellitus (DM)
- type 1 diabetes mellitus (T1DM)
- type 2 diabetes mellitus (T2DM)
- multiple endocrine neoplasia type 1 (MEN1; Young, 2022)
****DM, thyroid dysfunction, and osteoporosis are common disorders of the endocrine system and are discussed in depth in the NursingCE educational modules Diabetes, Thyroid Dysfunction, and Osteoporosis. They will not be explored in detail in this educational offering.
Anatomy and Physiology of the Endocrine System
The endocrine system consists of glands that produce and secrete hormones to regulate cell and organ activity as well as the body's growth, metabolism, sexual function, and development. The endocrine system communicates with the body through the secretion of hormones. These hormones are biochemical substances that bind to their specific target receptor sites. The hormones serve as the body's chemical messengers, which transfer information from one organ to another, coordinating functions between various body parts. The integral features of the endocrine system include the hypothalamus, pituitary, thyroid, adrenals, pancreas, parathyroids, pineal body, and ovaries/testes. Each endocrine gland secretes a set of hormones that help regulate the body's functions like a thermostat regulates the temperature in a building. Disorders, impairment, or imbalances of the endocrine system involve increases or decreases in hormone production or target cellular receptor failure (Assessment Technologies Institute [ATI], 2023). Figure 1 illustrates the endocrine system and its glands.
The endocrine system regulates itself based on a feedback system that involves stimulating hormones and releasing hormones. This feedback system maintains a balance of hormone levels within the bloodstream. Releasing hormones are sent from the hypothalamus to the pituitary, which prompts the pituitary to secrete various stimulating hormones. The stimulating hormones signal the target glands to release hormones into the systemic circulation. This feedback response, known as the hypothalamic-pituitary axis (HPA), is explained in detail throughout the module. There are different endocrine feedback response axes depending on the target gland (e.g., hypothalamic-pituitary-gonad axis, hypothalamic-pituitary-thyroid axis; Banasik & Copstead, 2019).
The Endocrine System
As shown in Figure 1, the thymus is positioned beneath the sternum and contains lymphatic tissue. It reaches its maximum size during puberty and then slowly atrophies. The thymus produces the hormones thymopoietin, thymulin, thymus humoral factor, and thymosin, which promote peripheral lymphoid tissue growth. The thymus's primary role is to produce T-lymphocytes (T-cells), crucial for cell-mediated immunity (Venes, 2021).
The hypothalamus appears within the lower central part of the brain above the pituitary gland and brain stem (see Figure 2). The key role of the hypothalamus is to maintain our body's homeostasis. It is comparable to the size of an almond and is fundamental for regulating body temperature, metabolism, and satiety. As mentioned, the hypothalamus controls the release of hormones from the pituitary gland and detects hormonal levels. The hypothalamus and the pituitary gland work collaboratively through a feedback mechanism known as the hypothalamic-pituitary axis or HPA. Through the axis (see Figure 3), signal or feedback is received from several areas of the central nervous system to the hypothalamus regarding hormone levels in the body, which sends a message to the pituitary gland to release or inhibit hormones in response (ATI, 2023).
Hypothalamus, Pituitary, and Pineal Gland Anatomy
The hypothalamus acts as a central coordinator for the entire endocrine system. Hypothalamic dysfunction can result from tumors
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As shown in Figure 2, the pituitary is about the size of a pea and located beneath the hypothalamus. Based on input from the hypothalamus and continuous monitoring of circulating hormone levels, the pituitary secretes hormones that regulate/control the function of the other endocrine glands. Hypopituitarism occurs if the pituitary does not produce a hormone or if it produces an insufficient amount. In contrast, hyperpituitarism occurs when the pituitary produces excess hormones (Hashim, 2020). Examples of the hormones regulated by the hypothalamic/pituitary system are listed in Table 1 and Figure 4.
The pituitary gland contains two parts, the anterior and posterior lobes, which produce several hormones. The pituitary's anterior lobe comprises different cell types: lactotrophs, somatotrophs, corticotrophs, gonadotrophs, thyrotrophs, and melanocytes. The posterior lobe of the pituitary is innervated by hypothalamic nerve cells and serves as a reservoir for releasing oxytocin and ADH (Hashim, 2020). See Table 1 and Figure 4 for the hormones produced in each area and their function.
Hypothalamic/Pituitary System Hormones and Their Function
Releasing Hormone Produced by the Hypothalamus
Stimulating Hormone Produced by the Anterior Pituitary
Function of Target Hormone
Thyrotropin-releasing hormone (TRH)
Thyroid-stimulating hormone (TSH)
Thyroid hormones (triiodothyronine [T3] and thyroxine [T4])
T3 (triiodothyronine)/T4 (thyroxine) influence metabolism and sympathetic nervous system activities.
Corticotropin-releasing hormone (CRH)
Adrenocorticotropic hormone (ACTH)
In response to stress, cortisol increases blood glucose (BG), cardiac output, and oxygen consumption.
Gonadotropin-releasing hormone (GnRH)
Follicle-stimulating hormone (FSH), luteinizing hormone (LH)
Estrogen, progesterone, testosterone
Estrogen, progesterone, and testosterone control sexual function and development. LH stimulates the development of the corpus luteum of the uterus and progesterone release. FSH stimulates the maturation of the ovarian follicles (females) or maintains spermatogenesis (males).
Growth hormone-releasing hormone (GHRH)
Growth hormone (GH)
Insulin-like growth factor-1 (IGF-1)
IGF-1 increases glucose uptake by muscle and adipose tissue throughout the body, stimulates protein synthesis in the liver, and inhibits the process of lipolysis (the breakdown of lipids by hydrolysis to release fatty acids).
Prolactin-releasing hormone (PRH)
Gonads and mammary glands
PRL stimulates mammary gland growth and development, as well as milk production following childbirth (see Figure 5)
Melanocyte-stimulating hormone (MSH)
Melanocytes in the skin
MSH stimulates the melanocytes in the skin and is responsible for the pigmentation of the skin.
Releasing Hormone Produced by the Hypothalamus
Stimulating Hormone Produced by the Posterior Pituitary
Function of stimulating hormone
Antidiuretic hormone (ADH)
ADH, or vasopressin, is stored in and excreted by the posterior pituitary. It regulates the kidney excretion of water as well as arterial vasoconstriction.
Mammary and muscles of the uterus
Oxytocin is also stored/released in the posterior pituitary. It contracts the uterus during childbirth and stimulates milk ejection (see Figure 5)
(Carmichael, 2023; El Sayed et al., 2023; Hashim, 2020; Shahid et al., 2023; Welt, 2023)
Pituitary Hormones and Their Target Sites
Hypothalamic-Pituitary Regulation of Prolactin and Oxytocin During Breastfeeding
The pineal gland—sometimes called the pineal body—is in the middle of the brain (see Figures 1 and 2). It is also known as the “spiritual third eye” or “third eye”, as research suggests that the pineal gland has neurons between the retina; light affects its secretion. This gland functions to maintain circadian rhythm (sleep-wake cycle) and timing and release of reproductive hormones through melatonin secretion. The production of melatonin is regulated by the hypothalamus and the pineal gland in response to signals from the retinal ganglion cells to regulate the sleep-wake cycle (Chaudhary et al., 2022).
Thyroid and Parathyroids
The thyroid gland is shaped like a small butterfly and is anterior to the trachea between the cricoid cartilage and suprasternal notch (Figures 1 and 6). It produces hormones that regulate metabolism. The hypothalamic-pituitary-thyroid axis regulates thyroid hormone production (Rivkees & Bauer, 2019; Young, 2022).
Thyroid and Parathyroid Glands
In children, thyroid hormones support bone growth, brain development, and nervous system development. In adults, they affect blood pressure (BP), heart rate (HR), muscle tone, and reproductive functions. The thyroid hormones also regulate body temperature, metabolism, and calcitonin, as well as impacting the way tissues outside the thyroid function. The thyroid gland generates two primary hormones: thyroxine (T4) and triiodothyronine (T3). As demonstrated in Figure 7, the release rate of T3 and T4 is controlled by the anterior pituitary gland and hypothalamus, which acts as a sensory controller. The process is initiated by the hypothalamus, which emits TRH. TRH prompts the production of TSH from the anterior pituitary gland. TSH binds to the receptors on the thyroid follicular cells, which causes the production and release of T3 and T4 (Rivkees & Bauer, 2019; Young, 2022). The amount of TSH that the pituitary releases into the bloodstream depends on the amount of T3 and T4 that the pituitary perceives, as it functions on a negative feedback loop system (see Figure 7). The pituitary constantly measures the amount of T3/T4 and responds to changes to maintain equilibrium. If the pituitary senses insufficient T4, it will boost TSH production, signaling the thyroid gland to produce more T4. Once T4 reaches an acceptable level in the blood, TSH production decreases (American Thyroid Association, n.d.). In circulation, T3 and T4 are bound to proteins such as thyroxine-binding globulin, albumin, and prealbumin (Rivkees & Bauer, 2019).
Hypothalamic-Pituitary-Thyroid Axis Feedback Mechanism
The parathyroid glands are embedded in the thyroid gland's surface, as shown in Figures 1 and 6. The two pairs of parathyroid glands release parathyroid hormone (PTH), which regulates serum calcium and phosphorus as well as bone metabolism. PTH extracts calcium from bones and reduces calcium excretion through the kidneys to increase serum calcium levels when too low. It also increases the production of calcitriol by converting calcidiol to calcitriol in the kidneys, which helps increase calcium absorption through the digestive system (Lofrese et al., 2023).
The adrenal glands are superior to the kidneys and comprise two parts: the medulla and cortex (see Figures 1 and 8). The adrenal cortex is the outer portion and consists of three specific zones: outer zona glomerulosa, middle zona fasciculata, and inner zona reticularis. Cortisol is produced in the middle zona in response to ACTH. The inner zona produces androgen (Banasik & Copstead, 2019).
The adrenal cortex produces corticosteroids (three types: mineralocorticoids, glucocorticoids, and androgens, see Table 2) that play a vital role in regulating metabolism, salt and water balance, the immune system, and sexual function (Grossman, 2022; Young, 2022). Steroids are soluble lipids produced on demand into systemic circulation bound to proteins such as albumin and transcortin (corticosteroid-binding globulin; Banasik & Copstead, 2019). Cortisol (a glucocorticoid) and androgens bind with their specific proteins, while aldosterone (a mineralocorticoid) does not. The adrenal gland is regulated by a negative feedback system involving the hypothalamus, which secretes CRH, triggering the corticotrophs in the anterior pituitary to secrete ACTH. Increasing levels of adrenal hormones in the circulation inhibit the secretion of both CRH and ACTH, completing the hypothalamus-pituitary-adrenal axis (El Sayed et al., 2023). The medulla—the inner part of the adrenal gland—produces catecholamines, including epinephrine (adrenaline) and norepinephrine (fight-or-flight hormones). Catecholamines help the body deal with emotional or physical stress by increasing HR and BP (Grossman, 2022; Young, 2022).
When the adrenal glands do not produce enough hormones, adrenal insufficiency or Addison's disease can result. Understanding normal cortisol physiology is important to understand adrenal dysfunction. Cortisol helps control the body's use of fats, proteins, and carbohydrates; suppresses inflammation; regulates BP via vascular tone (a decrease in cortisol production is associated with a decrease in BP); increases BG; and may decrease the formation of bone tissue. Cortisol impacts the body’s response to stress and stressful situations (cortisol is a fight-or-flight hormone). Increased cortisol is needed to cope with acutely stressful situations such as surgery. An inadequate release of cortisol in these situations can be fatal. Cortisol regulates the sleep/wake cycle and boosts energy during increased stress. Varying amounts of cortisol are secreted throughout the day: the lowest levels occur at midnight and the highest early in the morning (Grossman, 2022).
Steroids of the Adrenal Gland and Their Function
Type of Steroid
Maintain sodium and water balance by retaining sodium and excreting potassium in distal tubules of the kidneys in response to the renin-angiotensin system
Regulates blood pH
Assists with the metabolism of glucose and opposes the effect of insulin (increase BG levels)
Increase blood cholesterol levels
Protein catabolism by releasing proteins stored in muscles to provide amino acids for the production of glucose in the liver (gluconeogenesis)
Protection against stress
Regulate the inflammatory and immune responses
Reproductive (sex) hormones
Converts to form estrogen in the ovaries.
Converts to androgens such as testosterone in the testes.
Responsible for the development of secondary sex characteristics
Norepinephrine (“fight or flight”)
Sympathetic nervous system stimulation to assist with managing emotional/physical stress by increasing HR and BP
(Banasik & Copstead, 2019)
Testes and Ovaries
The gonads—testes and ovaries (see Figures 1 and 9)—are the reproductive glands and the main source of sex hormones. They are controlled by the hypothalamus-pituitary-gonadal (HPG) axis via the secretion of GnRH, LH, and FSH. The hormones primarily produced by the testes are called androgens; the most important is testosterone, which controls facial hair, pubic hair, and sexual development. In male patients, testosterone helps regulate spermatogenesis and libido (sexual desire), as well as muscle/bone mass in both sexes. In female patients, the ovaries produce progesterone, estrogen, and testosterone. These hormones control the development of secondary sexual characteristics (e.g., breasts, pubic hair), menstruation, libido, and ovulation. Estrogen also impacts libido, muscle/lipid mass, and fertility in males. These androgens and estrogen are excreted in smaller quantities by the adrenal gland (Endocrine Society, 2022; Young, 2022). Testosterone must first be converted to estradiol by aromatase to function properly, affect the gonads, and promote hair growth; it must be converted to dihydrotestosterone by the enzyme 5-alpha-reductase to act on bone tissue (Snyder, 2022).
Hypothalamic-Pituitary-Gonadal (HPG) Axis
The physiologic functioning of the reproductive system relies heavily on the appropriate secretion of numerous hormones. The HPG axis is central to this action, which involves the production of GnRH by the hypothalamus, followed by the anterior pituitary secretion of LH and FSH (Figure 9). LH and FSH bind to G protein-coupled receptors and trigger the secretion of androgens by the testes and estrogen and progesterone by the ovaries. LH is responsible for triggering the secretion of progesterone and testosterone by the ovaries. In premenopausal adolescent and adult females, a surge in LH also prompts ovulation. FSH triggers the secretion of estrogen and the process of ovum development. Estrogen levels are higher around ovulation and decrease during menstruation (El Sayed et al., 2023; Endocrine Society, 2022; Shahid et al., 2023).
In males, LH triggers testosterone secretion by the testes, and FSH facilitates spermatogenesis. Testosterone is crucial for developing secondary sexual characteristics during puberty, including penile/testicular enlargement, hair growth, voice deepening, increased bone and muscle mass, and increased skeletal height. Beyond puberty, testosterone production facilitates spermatogenesis, libido, and the maintenance of bone and muscle mass (El Sayed et al., 2023; Endocrine Society, 2022; Shahid et al., 2023).
The pancreas (see Figures 1 and 10) is located behind the stomach in the posterior portion of the abdomen. It has digestive and hormonal functions, termed endocrine (insulin) and exocrine (digestive enzymes). Insulin and glucagon are the two hormones secreted by the pancreas that aid in regulating blood glucose (see Figure 10). Dysfunction within the pancreas can lead to DM (Young, 2022).
Pancreas and Its Hormones
Hormonal Release Mechanisms
As mentioned previously, the endocrine system works on a negative feedback system. The hypothalamus and pituitary glands monitor target hormone levels continuously. When these levels decline, the hypothalamus secretes releasing hormones, which triggers the pituitary gland to secrete stimulating or pituitary hormones. As listed in Table 1, the pituitary hormones have numerous and varied effects on the human body. These trophic hormones trigger their target glands to increase hormone production. When target gland hormone levels increase, releasing and stimulating hormone secretion decreases. As mentioned previously, this process is known as the HPA. This mechanism can be complicated, as many of these hormones exhibit secondary effects. For example, PRL affects the hypothalamus-pituitary-gonadal axis, as increased circulating levels of PRL inhibit the hypothalamic secretion of GnRH, leading to the reduced secretion of LH/FSH by the pituitary and the various sex hormones (estrogen, progesterone, and androgens) by the target glands. TRH, primarily responsible for triggering the secretion of TSH by the anterior pituitary, also appears to facilitate the production of PRL (El Sayed et al., 2023; Shahid et al., 2023; Welt, 2023).
Chemical regulation occurs when endocrine glands not regulated by the pituitary gland are controlled by other substances that stimulate gland secretion. Examples include BG levels that regulate the release of glucagon and insulin by the pancreas and serum calcium levels, which affect the release of PTH by the parathyroid glands. The third method for hormonal control involves the central nervous system, which may provide sensory input through the hypothalamus or directly to one of the glands of the endocrine system, such as the regulation of melatonin production by the pineal gland in response to ambient light or the release of catecholamines by the adrenal gland in response to stress (El Sayed et al., 2023; Shahid et al., 2023; Welt, 2023).
Older adults usually experience changes to their endocrine system function as part of aging. Age-related changes of the adrenal cortex can include the following:
- declining sex hormones
- decreasing aldosterone levels, which can contribute to orthostatic hypotension
- increasing PTH, which may lead to osteoporosis
- rising FSH, LH, and norepinephrine levels
- changing glucose metabolism, such as increased BG when faced with physical or mental stress (Warde et al., 2023)
Endocrine System Dysfunction
Certain disorders of the endocrine system can be caused by hyposecretion, in which there is insufficient release of hormones. This could result from problems with the trophic hormones (ACTH, TSH), which cause the target gland to secrete less of the hormone. Alternatively, disorders of the endocrine system can be caused by hypersecretion, in which increased amounts of a hormone are released. Decreased responsiveness may also result from hormone resistance caused by a deficiency of the receptor cells, which presents with many manifestations of hyposecretion (Banasik & Copstead, 2019).
For learners who are eager to access additional content related to the endocrine system and endocrine disorders, please see the following NursingCE courses:
- Thyroid Dysfunction
- Sexual Dysfunction
American Thyroid Association. (n.d.). Thyroid function tests. Retrieved August 20, 2023, from https://www.thyroid.org/thyroid-function-tests/
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Assessment Technologies Institute. (2017b). Hypothalamic pituitary axis [image]. https://cms.ascendlearning.com/share/page/site/digital-asset-librarytest/document-details?nodeRef=workspace://SpacesStore/4b8521bf-c996-4ea5-a66e-5c94bde5ebcb
Assessment Technologies Institute. (2019a). The endocrine system [image]. https://cms.ascendlearning.com/share/page/site/digital-asset-librarytest/document-details?nodeRef=workspace://SpacesStore/39e2d9e3-e7cf-49d3-943f-efda9f22eab2
Assessment Technologies Institute. (2019b). Hypothalamic-pituitary-gonadal (HPG) axis [image]. https://cms.ascendlearning.com/share/page/site/digital-asset-librarytest/document-details?nodeRef=workspace://SpacesStore/97299daa-3f8c-47cf-8e98-b9f4b3ecd26a
Assessment Technologies Institute. (2019c). Hypothalamic-pituitary regulation of prolactin and oxytocin during breastfeeding [image]. https://cms.ascendlearning.com/share/page/site/digital-asset-librarytest/document-details?nodeRef=workspace://SpacesStore/448645c5-8f29-4e03-b9da-065c9ad1563d
Assessment Technologies Institute. (2019d). Hypothalamic-pituitary-thyroid axis feedback mechanism [image]. https://cms.ascendlearning.com/share/page/site/digital-asset-librarytest/document-details?nodeRef=workspace://SpacesStore/905fd39c-2cd1-4804-ac79-dc704a400a93
Assessment Technologies Institute. (2019e). Hypothalamus, pituitary, and pineal gland cross-section view of anatomy [image]. https://cms.ascendlearning.com/share/page/site/digital-asset-librarytest/document-details?nodeRef=workspace://SpacesStore/4d9ec632-b888-43ad-a05e-e0e69af8da6f
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Grossman, A. B. (2022). Overview of adrenal function. Merck Manual. https://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/adrenal-disorders/overview-of-adrenal-function
Hashim, I. A (2020). The clinical laboratory’s role in diagnosis and management of hypopituitarism. Medical Laboratory Observer, 51 (11), 8-11. https://www.mlo-online.com/continuing-education/article/21110537/the-clinical-laboratorys-role-in-diagnosis-and-management-of-hypopituitarism
Lofrese, J. J., Basit, H., & Lappin, S. L. (2023). Physiology, parathyroid. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK482510
Rivkees, S. A., & Bauer, A. J. (2019). Thyroid disorders: Manifestations, evaluation, and management in children and adolescents. Contemporary Pediatrics, 36(8), 33-41. https://www.contemporarypediatrics.com/view/thyroid-disorders-manifestations-evaluation-and-management-children-and-adolescents
Shahid, Z., Asuka, E., & Singh, G. (2023). Physiology, hypothalamus. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK535380/
Snyder, P. J. (2022). Testosterone treatment of male hypogonadism. UpToDate. Retrieved August 15, 2023, from https://www.uptodate.com/contents/testosterone-treatment-of-male-hypogonadism
Venes, D. (2021). Taber's cyclopedic medical dictionary (23rd ed.). F. A. Davis Company.
Warde, K. M., Smith, L. J., & Basham, K. J. (2023). Age-related changes in the adrenal cortex: Insights and implications. Journal of the Endocrine Society, 7(9). https://doi.org/10.1210/jendso/bvad097
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Young, W. F. (2022). Overview of endocrine system. Merck Manual. https://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/principles-of-endocrinology/overview-of-the-endocrine-system