The Endocrine System: Structure, Function, and Clinical Significance

The Endocrine System

Overview

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. These hormones control metabolism, growth, development, tissue function, sexual function, reproduction, sleep, and mood.

The Thyroid Gland

Location and Structure

The thyroid gland is a butterfly-shaped organ located in the anterior neck, just inferior to the larynx and on the trachea. It consists of two lateral lobes connected by a median mass called the isthmus. The gland contains follicles, which are hollow spheres of epithelial follicular cells that produce the glycoprotein thyroglobulin.

Colloid and Parafollicular Cells

The colloid is the fluid within the follicle lumen, containing thyroglobulin and iodine, serving as the precursor to thyroid hormone. Parafollicular cells, located outside the follicles, produce the hormone calcitonin.

Thyroid Hormone (TH)

Forms and Synthesis

Thyroid hormone is the body's major metabolic hormone and exists in two forms:

• T4 (thyroxine): Consists of two tyrosine molecules with four bound iodine atoms; the major form secreted by the thyroid.

• T3 (triiodothyronine): Consists of two tyrosines with three bound iodine atoms; more active at the tissue level.

Both are iodine-containing amine hormones. Synthesis involves several steps, including thyroglobulin production, iodide trapping, oxidation to iodine, attachment to tyrosine, and formation of T3 and T4.

Transport and Regulation

T4 and T3 are transported by thyroxine-binding globulins (TBGs). T3 is ten times more active than T4. Peripheral tissues convert T4 to T3 by removing one iodine atom. Release of TH is regulated by negative feedback: falling TH levels stimulate TSH release, while rising TH levels inhibit TSH.

Major Effects of Thyroid Hormone

• Increases basal metabolic rate and heat production (calorigenic effect)

• Regulates tissue growth and development, especially skeletal and nervous systems

• Maintains blood pressure by increasing adrenergic receptors in blood vessels

Process or System	Normal Effects	Effects of Hyposecretion	Effects of Hypersecretion
Metabolic rate	Promotes normal BMR	BMR below normal, cold intolerance	BMR above normal, heat intolerance
Carbohydrate/lipid/protein metabolism	Promotes glucose utilization	Impaired glucose metabolism	Enhanced catabolism of glucose
Cardiovascular system	Promotes normal heart function	Decreased heart function	Increased heart function
Nervous system	Promotes normal development	Impaired development	Hyperexcitability
Muscular system	Promotes normal muscular development	Muscle weakness	Muscle tremor
Gastrointestinal system	Promotes normal motility	Decreased motility	Increased motility
Reproductive system	Promotes normal reproductive function	Impaired function	Impaired function
Integumentary system	Promotes normal hydration	Dry, thick skin	Flushed, moist skin

Clinical Homeostatic Imbalances of Thyroid Hormone

Hyposecretion

• In adults, can lead to myxedema: low metabolic rate, thick/dry skin, puffy eyes, feeling chilled, constipation, edema, mental sluggishness, lethargy.

• If due to iodine deficiency, a goiter may develop: thyroid enlarges as it attempts to produce more hormone.

Congenital Hypothyroidism

• Usually caused by poor development of the thyroid gland in early childhood.

• May present with weak cry, poor feeding, constipation, or prolonged jaundice.

• TH replacement is crucial and lifelong.

Hypersecretion

• Most common type is Graves’ disease: autoimmune disorder where antibodies mimic TSH, stimulating TH release.

• Symptoms: elevated metabolic rate, sweating, rapid and irregular heartbeats, nervousness, weight loss, exophthalmos (bulging eyes).

• Treatment: surgical removal or radioactive iodine.

Calcitonin

• Produced by parafollicular (C) cells in response to high Ca2+ levels.

• Antagonist to parathyroid hormone (PTH).

• At high doses, inhibits osteoclast activity and stimulates Ca2+ uptake into bone matrix.

The Parathyroid Glands

Location and Structure

Four to eight tiny yellow-brown glands embedded in the posterior aspect of the thyroid. Contain oxyphil cells (function unclear) and parathyroid cells that secrete parathyroid hormone (PTH), the most important hormone in Ca2+ homeostasis.

Functions of Parathyroid Hormone (PTH)

• Stimulates osteoclasts to digest bone matrix and release Ca2+ to blood.

• Enhances reabsorption of Ca2+ and secretion of phosphate by kidneys.

• Promotes activation of vitamin D by kidneys, increasing absorption of Ca2+ by intestinal mucosa.

Clinical Homeostatic Imbalances of Parathyroid Hormone

Hyperparathyroidism

• Usually due to parathyroid gland tumor.

• Calcium leaches from bones, causing them to soften and deform.

• Elevated Ca2+ depresses nervous system and contributes to kidney stones.

• Osteitis fibrosa cystica: severe form resulting in easily fractured bones.

Hypoparathyroidism

• Following gland trauma or removal can cause hypocalcemia.

• Results in tetany, respiratory paralysis, and death.

The Adrenal Glands

Location and Structure

Paired, pyramid-shaped organs atop the kidneys, also called suprarenal glands. Structurally and functionally two glands in one:

• Adrenal cortex: Three layers of glandular tissue synthesizing and secreting corticosteroids.

• Adrenal medulla: Nervous tissue, part of the sympathetic nervous system.

Adrenal Cortex

Mineralocorticoids

• Regulate electrolyte concentrations (primarily Na+ and K+) in extracellular fluid.

• Aldosterone: Most potent mineralocorticoid; stimulates Na+ reabsorption and K+ elimination by kidneys.

• Regulated by renin-angiotensin-aldosterone mechanism, plasma K+ concentration, ACTH, and atrial natriuretic peptide (ANP).

Clinical Imbalance: Aldosteronism

• Hypersecretion usually due to adrenal tumors.

• Results in hypertension, edema, and excretion of K+, leading to abnormal neurons and muscle function.

Glucocorticoids

• Influence metabolism of most cells and help resist stressors.

• Keep blood glucose levels constant and maintain blood pressure.

• Cortisol: Only glucocorticoid in significant amounts in humans; released in response to ACTH.

• Prime metabolic effect: gluconeogenesis (formation of glucose from fats and proteins).

• Excessive levels depress cartilage/bone formation, inhibit inflammation, depress immune system, and disrupt normal functions.

Clinical Imbalances

• Cushing’s syndrome: Hypersecretion; causes "moon" face, "buffalo hump," immune suppression, and other disruptions.

• Addison’s disease: Hyposecretion; deficits in glucocorticoids and mineralocorticoids, weight loss, dehydration, hypotension, and skin bronzing.

Gonadocorticoids

• Weak androgens converted to testosterone or estrogens.

• Contribute to puberty, sex drive in women, and source of estrogens in postmenopausal women.

• Hypersecretion leads to adrenogenital syndrome (masculinization).

Adrenal Medulla

• Medullary chromaffin cells synthesize catecholamines: epinephrine (80%) and norepinephrine (20%).

• Effects: vasoconstriction, increased heart rate, increased blood glucose, blood diverted to brain, heart, and skeletal muscle.

• Epinephrine stimulates metabolic activities; norepinephrine influences peripheral vasoconstriction and blood pressure.

• Responses to stressors are brief.

Clinical Imbalance

• Hypersecretion leads to symptoms of uncontrolled sympathetic nervous system (hyperglycemia, rapid heartbeat, hypertension, nervousness, sweating).

• Can be due to pheochromocytoma (tumor of medullary chromaffin cells).

Hormone	Regulation of Release	Target Organ and Effects	Effects of Hyper/Hyposecretion
Mineralocorticoids (aldosterone)	Stimulated by renin-angiotensin mechanism, increased K+, ACTH, inhibited by ANP	Kidney: increases blood Na+, decreases blood K+, increases blood volume and pressure	Hyper: Aldosteronism; Hypo: Addison's disease
Glucocorticoids (cortisol)	Stimulated by ACTH, inhibited by feedback	Body cells: promotes gluconeogenesis, resists stress, depresses immune system	Hyper: Cushing's syndrome; Hypo: Addison's disease
Gonadocorticoids (androgens)	Stimulated by ACTH	Reproductive organs: onset of puberty, sex drive	Hyper: Masculinization; Hypo: No effects
Adrenal medullary hormones (epinephrine, norepinephrine)	Stimulated by preganglionic sympathetic fibers	Heart, blood vessels, liver: fight-or-flight response	Hyper: Prolonged fight-or-flight; Hypo: No effects

The Pineal Gland

Structure and Function

Small gland hanging from the roof of the third ventricle. Pinealocytes secrete melatonin, derived from serotonin. Melatonin affects timing of sexual maturation, day/night cycles, physiological rhythms, and production of antioxidant molecules.

The Pancreas

Location and Structure

Triangular gland located partially behind the stomach. Contains both exocrine (acinar cells) and endocrine (islets of Langerhans) cells.

Hormones of the Pancreas

• Alpha (α) cells: Produce glucagon (hyperglycemic hormone).

• Beta (β) cells: Produce insulin (hypoglycemic hormone).

Glucagon

• Triggered by decreased blood glucose, rising amino acids, or sympathetic stimulation.

• Raises blood glucose by targeting the liver for glycogenolysis and gluconeogenesis.

Insulin

• Secreted when blood glucose increases.

• Lowers blood glucose by enhancing transport into fat and muscle cells, inhibiting glycogen breakdown, and inhibiting conversion of amino acids/fats to glucose.

• Triggers cells to oxidize glucose for ATP, polymerize glucose to glycogen, and convert glucose to fat.

Clinical Homeostatic Imbalances of Pancreatic Hormones

Diabetes Mellitus (DM)

• Type 1: Hyposecretion of insulin

• Type 2: Hypoactivity of insulin

• Three cardinal signs: polyuria (excess urine), polydipsia (excess thirst), polyphagia (excess hunger)

• Fats used as fuel, causing lipidemia and formation of ketones (ketoacidosis)

• Untreated ketoacidosis can lead to coma and death

Hyperinsulinism

• Excessive insulin secretion causes hypoglycemia (low blood glucose)

• Symptoms: anxiety, nervousness, disorientation, unconsciousness, death

• Treatment: sugar ingestion

The Gonads and Placenta

Ovaries

• Produce estrogens and progesterone

• Estrogen: maturation of reproductive organs, secondary sexual characteristics

• Progesterone: breast development, cyclic changes in uterine mucosa

Testes

• Produce testosterone

• Initiates maturation of male reproductive organs, secondary sexual characteristics, sex drive

• Necessary for normal sperm production

Placenta

• Secretes estrogens, progesterone, and human chorionic gonadotropin (hCG)

Hormone Secretion by Other Organs

Adipose Tissue

• Leptin: appetite control

• Resistin: insulin antagonist

• Adiponectin: enhances sensitivity to insulin

Gastrointestinal Tract

• Gastrin: stimulates HCl release

• Ghrelin: stimulates food intake

• Secretin: stimulates liver and pancreas

• Cholecystokinin (CCK): activates pancreas, gallbladder

• Incretins: enhance insulin release, inhibit glucagon

Heart

• Atrial natriuretic peptide (ANP): decreases blood Na+, blood pressure, and blood volume

Kidneys

• Erythropoietin: signals production of red blood cells

• Renin: initiates renin-angiotensin-aldosterone mechanism

Skeleton

• Osteocalcin: prods pancreas to secrete more insulin, restricts fat storage, improves glucose handling

Skin

• Cholecalciferol: precursor of vitamin D

• Calcitriol: active form of vitamin D, helps absorb calcium, modulates immunity

Thymus

• Thymulin, thymopoietins, thymosins: involved in T lymphocyte development