21.1: Review of Endocrine Anatomy and Physiology

Learning Objectives

By the end of this section, you will be able to:

• Explain the anatomy of the endocrine system
• Discuss common endocrine hormones and their functions
• Summarize the steps of a targeted endocrine system assessment

The endocrine system is responsible for regulating major biological processes in the body, such as digestion, metabolism, and the stress response, but is often overlooked. It is similar to the nervous system because it can send signals throughout the body, but instead of neurotransmitters, it releases hormones as the mode of communication. Disruptions to this communication process can cause major problems with the biologic functions of the body. As a nurse, it is important to have a basic understanding of the endocrine system in order to provide quality patient care.

Overview of the Endocrine System

The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function. Among the most important of these structures are the endocrine glands, including the pituitary, thyroid, parathyroid, adrenal, and pineal glands (Figure 21.2). The primary function of these ductless glands is to secrete their hormones directly into the surrounding fluid. The interstitial fluid and the blood vessels then transport the hormones throughout the body.

Figure 21.2 Endocrine glands and cells are located throughout the body and play an important role in homeostasis. (modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Some glands in the endocrine system have both endocrine and non-endocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, ovaries, and testes are other organs that contain cells with endocrine function. Adipose tissue (body fat) has long been known for producing hormones that help regulate various bodily functions. Recent research has expanded our understanding by revealing that bone tissue also has endocrine functions, meaning it can produce hormones that influence metabolism and other physiological processes (Newman, 2024).

The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through tubes called ducts. Examples of exocrine glands include the sebaceous and sweat glands of the skin. The pancreas also has an exocrine function: most of its cells secrete pancreatic juice through the pancreatic and accessory ducts to the lumen of the small intestine.

Neural and Endocrine Signaling

The nervous system uses two types of intercellular communication—electrical and chemical signaling—either by the direct action of an electrical potential, or in the latter case, through the action of chemical neurotransmitters such as serotonin or norepinephrine. Neurotransmitters act locally and rapidly. When an electrical signal in the form of an action potential arrives at the synaptic terminal, they diffuse across the synaptic cleft (the gap between a sending neuron and a receiving neuron or muscle cell). Once the neurotransmitters interact (bind) with receptors on the receiving (post-synaptic) cell, the receptor stimulation is transduced into a response such as continued electrical signaling or modification of cellular response. The target cell responds within milliseconds of receiving the chemical “message;” this response then ceases very quickly once the neural signaling ends. In this way, neural communication enables body functions that involve quick, brief actions such as movement, sensation, and cognition.

In contrast, the endocrine system uses just one method of communication: chemical signaling. These signals are sent by the endocrine organs, which secrete chemicals—the hormones—into the extracellular fluid. Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, inducing a characteristic response. As a result, this process, known as endocrine signaling, requires more time than neural signaling to prompt a response in target cells, though the precise amount of time varies with different hormones. For example, the hormones released when you are confronted with a dangerous or frightening situation, called the fight-or-flight response, occur by the release of adrenal hormones—epinephrine and norepinephrine—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones.

In addition, endocrine signaling is typically less specific than neural signaling. The same hormone may play a role in a variety of different physiological processes depending on the target cells involved. For example, the hormone oxytocin promotes uterine contractions in people in labor. It is also important in breastfeeding and may be involved in the sexual response and in feelings of emotional attachment in humans.

In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction. So how does the fight-or-flight response happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary.

Hormones

A chemical substance called a hormone travels throughout the body in the bloodstream and affects the activity only of their target cells: that is, cells with receptors for that particular hormone. Once a hormone binds to a receptor, a chain of events is initiated that leads to the target cell’s response. Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. These responses contribute to reproduction, growth and development of body tissues, metabolism, fluid and electrolyte balance, sleep, and many other body functions. The major hormones of the human body and their effects are identified in (Table 21.1).

Endocrine Gland	Associated Hormones	Effect
Pituitary (anterior)	Growth hormone (GH)	Promotes growth of body tissues
	Prolactin (PRL)	Promotes milk production
	Thyroid-stimulating hormone (TSH)	Stimulates thyroid hormone release
	Adrenocorticotropic hormone (ACTH)	Stimulates hormone release by adrenal cortex
	Follicle-stimulating hormone (FSH)	Stimulates gamete production
	Luteinizing hormone (LH)	Stimulates androgen production by gonads
Pituitary (posterior)	Antidiuretic hormone (ADH)	Stimulates water reabsorption by kidneys
	Oxytocin	Stimulates uterine contractions during childbirth
Thyroid	Thyroxine (T4), triiodothyronine (T3)	Stimulate basal metabolic rate
	Calcitonin	Reduces calcium (Ca^2+) levels in the blood
Parathyroid	Parathyroid hormone (PTH)	Increases calcium (Ca^2+) levels in the blood
Adrenal (cortex)	Cortisol, corticosterone, cortisone	Increase blood glucose levels
Adrenal (medulla)	Epinephrine, norepinephrine	Stimulate fight-or-flight response
Pineal	Melatonin	Regulates sleep cycles
Pancreas	Insulin	Reduces blood glucose levels
	Glucagon	Increases blood glucose levels
Testes	Testosterone	Stimulates development of male sex characteristics including a deeper voice, increased muscle mass, development of body hair, and sperm production
Ovaries	Estrogen, progesterone	Stimulate development of female sex characteristics, including the development of adipose and breast tissue, and prepare the body for childbirth

Table 21.1 Endocrine Glands and Their Major Hormones

Hormones are divided into two major groups based on their chemical structure. Hormones derived from amino acids include amines, peptides, and proteins. Hormones derived from lipids include steroids. These chemical groups affect a hormone’s distribution, the type of receptors it binds to, and other aspects of its function.

Link to Learning

Watch this video to gain further understanding about endocrine glands and their functions as an NCLEX review.

Pathways of Hormone Action

The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor processes the message by initiating other signaling events or cellular mechanisms that result in the target cell’s response. Hormone receptors recognize molecules with specific shapes and side groups and respond only to those hormones that are recognized. For example, cells contain insulin receptors and glucose channels. A cell’s glucose channel will not open to accept glucose until insulin unlocks the insulin receptor, and only insulin will “fit” into the insulin receptor. The process is similar to using a specific key to unlock a door before opening it (Figure 21.3).

Figure 21.3 Insulin regulates the body’s metabolism and is the “key” that “unlocks” a cell’s glucose channel. (CC BY 4.0; Rice University & OpenStax)

Once the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, or stimulation of the secretion of products. A single hormone may be capable of inducing different responses in a given cell. Moreover, the same type of receptor may be located on cells in different body tissues and trigger somewhat different responses in each location. Thus, the response triggered by a hormone depends not only on the hormone but also on the target cell.

Regulation of Hormone Secretion

To prevent abnormal hormone levels and a potential disease state, hormone levels must be tightly controlled. The body maintains this control by balancing hormone production and degradation, or breakdown. Feedback loops govern the initiation and maintenance of most hormone secretion in response to various stimuli.

Role of Feedback Loops

A positive feedback loop is characterized by the release of additional hormone in response to an original hormone release. For example, the release of oxytocin during childbirth is a positive feedback loop. The initial release of oxytocin begins to signal the uterine muscles to contract, which pushes the fetus toward the cervix, causing it to stretch. This, in turn, signals the pituitary gland to release more oxytocin, causing labor contractions to intensify. The release of oxytocin decreases after the birth of the child.

The more common method of hormone regulation is the negative feedback loop. A negative feedback loop is characterized by the inhibition of further secretion of a hormone in response to adequate levels of that hormone. This allows blood levels of the hormone to be regulated within a narrow range. An example of a negative feedback loop is the release of glucocorticoid hormones from the adrenal glands, as directed by the hypothalamus and pituitary gland. As glucocorticoid concentrations in the blood rise, the hypothalamus and pituitary gland reduce their signaling to the adrenal glands, preventing additional glucocorticoid secretion (Figure 21.4).

Figure 21.4 The release of adrenal glucocorticoids is stimulated by the release of hormones from the hypothalamus and pituitary gland. This signaling is inhibited when glucocorticoid levels become elevated, a state that sends negative signals to the pituitary gland and hypothalamus. (modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Role of Endocrine Gland Stimuli

Reflexes triggered by both chemical and neural stimuli control endocrine activity. These reflexes may be simple, involving only one hormone response, or they may be more complex and involve many hormones, as is the case with the hypothalamic control of various anterior pituitary-controlled hormones.

Changes in blood levels of nonhormone chemicals, known as humoral stimuli, such as nutrients or ions, cause the release or inhibition of a hormone to, in turn, maintain homeostasis. For example, osmoreceptors in the hypothalamus detect changes in blood osmolarity (the concentration of solutes in the blood plasma). If blood osmolarity is too high, the blood is not diluted enough, so osmoreceptors signal the hypothalamus to release antidiuretic hormone (ADH). The hormone causes the kidneys to reabsorb more water and reduce the volume of urine produced. This reabsorption causes a reduction of the osmolarity of the blood, diluting the blood to the appropriate level. The regulation of blood glucose is another example. High levels of blood glucose cause the release of insulin from the pancreas, which increases glucose uptake by cells and liver storage of glucose as glycogen.

An endocrine gland may also secrete a hormone in response to the presence of another hormone produced by a different endocrine gland. Such hormonal stimuli often involve the hypothalamus, which produces releasing and inhibiting hormones that control the secretion of a variety of pituitary hormones.

In addition to these chemical signals, hormones can also be released in response to neural stimuli. A common example of neural stimuli is the activation of the fight-or-flight response by the sympathetic nervous system. When an individual perceives danger, sympathetic neurons signal the adrenal glands to secrete norepinephrine and epinephrine. The two hormones dilate blood vessels, increase the heart and respiratory rate, and suppress the digestive and immune systems. These responses boost the body’s transport of oxygen to the brain and muscles, thereby improving the body’s ability to fight the threat or flee from it.

Assessment of the Endocrine System

Clinical assessment of the endocrine system is a process that incorporates a patient history, physical examination, and diagnostic testing. The metabolic functions of endocrine glands can be monitored through the evaluation of the hormones they release.

Patient History

A patient history can provide targeted subjective data to the nurse and provider to evaluate the endocrine health of the patient. Table 21.2 lists questions that might be included in such a history.

Category	Focused Questions
Current health	What are your current goals for your health? Are there any other issues affecting your current health or the ability to complete your daily activities? If yes, tell me more. Are there any symptoms such as unexplained weight changes, fatigue, or changes in mood that concern you? If yes, please describe them in detail.
Medications	What are your current medications, including prescriptions, over-the-counter medications, vitamins, and herbal supplements, and why are you taking them? (Note: This is to establish the patient’s understanding of their medications.) Do you take your medications as prescribed? (Note: If the response is “no” or “sometimes,” follow up with an open-ended question such as, “Tell me more about the reasons for not taking the medications as prescribed.”)
Allergies	Do you have any allergies to medications, food, latex, or other items? (Note: If yes, ask the patient to describe the allergic reaction.)
Childhood illnesses	Tell me about any significant childhood illnesses that you had. Do you recall what childhood vaccines you received? When did these illnesses occur? Were you hospitalized? Did you experience any complications?
Family health history	Tell me about the health of your blood relatives. Does anyone have diseases like cancer, thyroid problems, heart problems, diabetes, or respiratory problems? Have any of your blood relatives died? If so, do you know the cause of death? What age did they die?
Chronic illnesses	Tell me about any chronic illnesses you currently have or have experienced (such as cancer, cardiac or respiratory issues, diabetes, or arthritis). When were you diagnosed? Do you see a specialist for this chronic illness? If so, what is their name and location? How is this condition currently being treated? How has the chronic illness affected you? How do you cope with it? Have you experienced any complications or disability from this chronic illness? If so, tell me more.
Acute illnesses, surgeries, accidents, or injuries	Tell me about any acute illnesses or surgeries that you have experienced. Have you had any accidents or injuries? Did you experience any complications?
Reproductive health	For females: When was your last menstrual period? Have you ever been pregnant? Are you pregnant now, or is there any chance of being pregnant now? Tell me about your pregnancies. Were there any issues or complications? For females of age: At what age did you first experience symptoms of menopause? Perimenopause? For males: Have you ever experienced any testicular disorders? Impotence or sexual dysfunction?

Table 21.2 Sample Focused Questions for Current and Past Health History

Physical Examination

Endocrine conditions can be nonspecific and varied, and they can affect many body systems. Apart from thyroid disease, diabetes mellitus, and some reproductive disorders, they are also relatively uncommon. Table 21.3 lists possible medical diagnoses of common clinical features seen in endocrine conditions (Crafa et al., 2022).

Sign or Symptom	Possible Medical Diagnoses
Altered facial appearance	Cushing’s syndrome, polycystic ovary syndrome (PCOS), acromegaly, hypothyroidism, dwarfism
Bone fragility	Hyperthyroidism, hypogonadism, Cushing’s syndrome
Delayed puberty	Hypothyroidism, hypopituitarism, primary gonadal failure, polycystic ovary syndrome
Diffuse neck swelling	Hashimoto’s thyroiditis, simple goiter, Graves’ disease
Erectile dysfunction	Diabetes mellitus, primary or secondary hypogonadism, non-endocrine systemic disease
Excessive thirst	Conn’s syndrome, diabetes mellitus or insipidus, hyperparathyroidism
Flushing	Carcinoid syndrome, hypogonadism, perimenopause, menopause
Hirsutism (excessive hair growth)	PCOS, Cushing’s syndrome, congenital adrenal hyperplasia
Menstrual disturbance	Thyroid dysfunction, PCOS, hyperprolactinemia
Muscle weakness	Osteomalacia (soft bones), hyperthyroidism, hyperparathyroidism, Cushing’s syndrome
Resistant hypertension	Renal artery stenosis, acromegaly, Cushing’s syndrome, Conn’s syndrome, phaeochromocytoma
Skin pigmentation changes (e.g., hyperpigmentation, bruising, pigmented spots)	Lentiginosis, primary adrenal insufficiency, Cushing’s syndrome, Addison’s disease, diabetes mellitus type I
Sweating	Acromegaly, hyperthyroidism, hypogonadism, phaeochromocytoma
Weight gain	Cushing’s syndrome, hypothyroidism, PCOS
Weight loss	Adrenal insufficiency, hyperthyroidism, diabetes mellitus

Table 21.3 Common Clinical Features in Endocrine Conditions

Link to Learning

Watch this video to review common endocrine disorders and their common clinical features.

When completing a physical assessment, inspect the patient’s neck for asymmetry, redness, swelling, surgical scars, or masses by having the patient turn their head slowly from side to side. Ask the patient to take a sip of water, tilt their head back to elongate the neck, and swallow. Watch for any lumps or protrusions as the patient swallows (Alomari, n.d.). Movement of the neck should be uniform and symmetrical when swallowing.

Palpate the neck for symmetry, noting any masses. Standing behind the patient, find and palpate the thyroid gland, using a circular motion, and note any enlargement, asymmetry, or masses. Palpate for any tracheal deviation or cervical lymph nodes. Auscultate each lobe of the thyroid for any bruits.

Link to Learning

Click here to watch how to perform a full clinical examination of the thyroid gland from introduction through palpation of the thyroid.

Have the patient extend their arms, place their palms up, and assess for any minor tremors or sweaty palms. Check the patients’ eyes to see if they bulge, a condition known as exophthalmos (Figure 21.5).

Figure 21.5 Exophthalmos is a clinical sign of an endocrine disorder. (credit: “Proptosis and lid retraction from Graves' Disease” by Jonathan Trobe, M.D., University of Michigan Kellogg Eye Center/Wikipedia, CC BY 3.0)

Diagnostic Testing

Diagnostic testing is targeted based on the findings of a history and physical exam. Serum hormone levels can assess under- or over-activity. Blood glucose and hemoglobin A1C (HbA1c) levels can determine diabetes mellitus. Urinalysis can evaluate for diabetes mellitus, urine electrolytes, and kidney damage. Imaging may be performed to visualize the physical appearance of different endocrine glands and organs and evaluate them for tumors. Fine needle biopsy may also be performed to assess any nodules for malignancies. Table 21.4 lists common diagnostic tests for endocrine assessment.

Diagnostic Test	Indication/Significance
Laboratory Testing
Urinalysis	Glycosuria (excess glucose in the urine) or proteinuria may indicate diabetes mellitus and/or kidney disease
Blood glucose	High levels indicate diabetes mellitus
Serum calcium	High levels indicate hyperparathyroidism Low levels indicate hypoparathyroidism
Serum cortisol	Low levels indicate hypoadrenalism High levels indicate Cushing’s disease
Gonadotrophins	High levels indicate primary hypogonadism
Imaging
CT scan	View the adrenal glands and pancreas
MRI	View the pancreas and pituitary gland
Ultrasound	View the thyroid, parathyroid, testes, and ovaries
Positron emission tomography (PET) scan	Evaluate thyroid and neuroendocrine tumors
Radionuclide imaging	Uses an isotope to better assess endocrine function, circulation, and tumors
Invasive Procedures
Inferior petrosal sinus sampling	Evaluate for a pituitary tumor by sampling adrenocorticotropic hormone (ACTH) from the veins that drain from the pituitary gland
Fine needle aspiration biopsy	Evaluate the cytology of a thyroid or adrenal nodule

Table 21.4 Targeted Diagnostic Testing in Endocrine Assessment