Unit 9: The Endocrine System

A comprehensive vocabulary review of endocrine anatomy, hormone characteristics, feedback mechanisms, gland histology, regulatory pathways, and associated pathophysiology.

Endocrine Gland

A ductless gland composed of cells that secrete chemical messengers called hormones directly into the blood to be transported to target tissues.

Exocrine Gland

A gland with ducts that carry its secretory products to the outside of the body or into a hollow organ, such as salivary glands secreting saliva or sweat glands secreting sweat.

Half-Life (Hormone)

The length of time required for the concentration of a hormone in the bloodstream to be reduced by half, reflecting hormone stability.

Water-Soluble Hormones

Hormones that circulate as free hormones dissolved directly into blood plasma and typically have short half-lives due to rapid degradation by hydrolytic enzymes.

Lipid-Soluble Hormones

Hormones that travel through the bloodstream bound to specialized binding proteins to reach their target cells.

Humoral Stimuli

The control of hormone release in response to fluctuating concentrations of specific substances or ions within body fluids such as blood (for example, blood $Ca^{2+}$ regulating parathyroid hormone release).

Neural Stimuli

The release of hormones stimulated by action potentials that cause neurons to secrete neurotransmitters onto endocrine cells (such as sympathetic stimulation releasing epinephrine from the adrenal medulla during stress).

Hormonal Stimuli

A control mechanism in which hormones stimulate or inhibit the secretion of other hormones (for example, tropic hormones from the anterior pituitary acting on target endocrine glands).

Tropic Hormones

Hormones that specifically target other endocrine glands and regulate their hormone synthesis and secretion.

Negative Feedback

A self-limiting regulatory mechanism wherein a hormone's secretion is inhibited by the hormone itself once its concentration in the blood reaches a sufficient level to activate target cells.

Positive Feedback

A self-propagating control system wherein stimulation promotes further synthesis and secretion of the tropic hormone, stimulating even greater release of the original hormone.

Adenohypophysis

The anterior lobe of the pituitary gland, which synthesizes and secretes hormones under the control of hypothalamic releasing and inhibiting hormones delivered via the hypothalamohypophysial portal system.

Neurohypophysis

The posterior lobe of the pituitary gland, which is continuous with the hypothalamus and stores neurohormones conducted along the hypothalamohypophysial tract.

Antidiuretic Hormone (ADH)

Also called vasopressin; a small peptide neurohormone from the posterior pituitary that targets the kidneys to increase water reabsorption, reducing urine volume, and constricts blood vessels to raise blood pressure.

Oxytocin

A small peptide neurohormone stored in the posterior pituitary that stimulates uterine contractions during labor and milk expulsion from the mammary glands.

Growth Hormone (GH)

Also known as somatotropin; an anterior pituitary protein hormone that stimulates growth in most tissues, promotes protein synthesis and lipid breakdown, increases liver glucose production, and exhibits daily peak levels during deep sleep.

Gigantism

A condition resulting from the chronic hypersecretion of growth hormone (GH) before the epiphyseal plates of long bones have ossified.

Acromegaly

A condition caused by elevated growth hormone (GH) levels occurring after epiphyseal plates have ossified, resulting in no height increase but an increased diameter of the hands, feet, and facial structures.

Pituitary Dwarfism

A condition characterized by delayed bone growth and short stature resulting from insufficient amounts of growth hormone (GH).

Thyroid-Stimulating Hormone (TSH)

Also known as thyrotropin; an anterior pituitary glycoprotein that stimulates the synthesis and secretion of thyroid hormones ($T_3$ and $T_4$) from the thyroid gland.

Adrenocorticotropic Hormone (ACTH)

An anterior pituitary peptide hormone that stimulates the adrenal cortex to secrete glucocorticoid hormones, primarily cortisol.

Thyroglobulin (Tg)

A large protein synthesized in thyroid follicle cells and secreted into the lumen of thyroid follicles, serving as the iodinated precursor to thyroid hormones.

Triiodothyronine (T3) and Tetraiodothyronine (T4)

Thyroid hormones that increase the metabolic rate of tissues by $60\%-100\%$, enhance $Na^+-K^+$ pump activity to generate heat, promote protein synthesis, and support organ growth and maturation.

Calcitonin

A hormone secreted by the parafollicular cells (C cells) of the thyroid gland when blood $Ca^{2+}$ is elevated, acting to decrease blood calcium levels by reducing bone resorption and lowering intestinal/renal uptake.

Parathyroid Hormone (PTH)

A hormone secreted by the parathyroid glands in response to low blood $Ca^{2+}$ that stimulates osteoclasts, increases kidney and intestinal calcium absorption, and raises blood calcium levels.

Goiter

An enlargement of the thyroid gland presenting as an anterior neck mass, which can develop from hyperthyroidism, loss of negative feedback in hypothyroidism, or iodine deficiency.

Graves Disease

An autoimmune form of hyperthyroidism in which patients produce thyroid-stimulating immunoglobulin (TSI) that mimics TSH, often manifesting with goiter and exophthalmos.

Exophthalmos

A clinical sign of hyperthyroidism characterized by protruding eyes caused by connective tissue proliferation and fluid deposits behind the eyes.

Adrenal Medulla

The inner core of the adrenal gland, consisting of neuroendocrine tissue that secretes epinephrine and norepinephrine to prepare the body for physical activity.

Adrenal Cortex

The outer region of the adrenal gland composed of three layers: the zona glomerulosa (mineralocorticoids), zona fasciculata (glucocorticoids), and zona reticularis (androgens).

Aldosterone

The major mineralocorticoid produced by the zona glomerulosa of the adrenal cortex that acts on the kidneys to increase sodium reabsorption, promoting water retention and elevating blood pressure.

Cortisol

The primary glucocorticoid produced by the zona fasciculata of the adrenal cortex that stimulates gluconeogenesis, elevates blood glucose, increases lipolysis, and suppresses inflammatory and immune reactions.

Pancreatic Islets

Also called Islets of Langerhans; endocrine cell clusters in the pancreas comprising alpha (glucagon-secreting), beta (insulin-secreting), and delta (somatostatin-secreting) cells.

Insulin

A peptide hormone secreted by pancreatic beta cells in response to elevated blood glucose, stimulating glucose uptake into body cells and promoting glycogen synthesis in the liver and skeletal muscle.

Glucagon

A peptide hormone secreted by pancreatic alpha cells in response to falling blood glucose levels that stimulates glycogenolysis and gluconeogenesis in the liver to increase circulating glucose.

Somatostatin (Pancreatic)

A peptide hormone produced by delta ($\delta$) cells of the pancreatic islets that inhibits the secretion of both insulin and glucagon.

Type 2 Diabetes Mellitus (T2DM)

A metabolic disorder caused by insulin resistance, characterized by impaired glucose uptake into cells, postprandial hyperglycemia, polyuria, polydipsia, fatigue, and poor wound healing.

Hemoglobin A1c (HbA1c)

A diagnostic blood test that measures the percentage of hemoglobin glycated by glucose, reflecting average blood glucose control over the preceding 3 months.

Melatonin

An indoleamine hormone secreted by the pineal gland during darkness that enhances the tendency to sleep and inhibits gonadotropin-releasing hormone (GnRH) release from the hypothalamus.

Arginine Vasotocin

A hormone secreted by the pineal gland that operates alongside melatonin to regulate reproductive system function.

Endocrine Gland

A ductless gland or specialized cell group that secretes chemical messengers (hormones) directly into the blood or bodily fluids to be transported to target effectors.

Exocrine Gland

A gland that possesses ducts that carry its secretions to the exterior of the body or into the lumen of a hollow organ (such as salivary glands producing saliva or sweat glands producing sweat).

Hormone Half-Life

The length of time required for the concentration of a hormone in the bloodstream to be reduced by half, serving as a measure of its stability and duration in circulation.

Water-Soluble Hormones

Free hormones that dissolve directly into the blood plasma and generally possess short half-lives because they are rapidly degraded by hydrolytic enzymes.

Lipid-Soluble Hormones

Non-polar hormones that travel in the bloodstream bound to specialized binding proteins, preventing rapid degradation and excretion.

Humoral Stimuli

A control mechanism in which hormone secretion is directly triggered by changes in the concentration of specific molecules, ions, or substances in body fluids such as blood.

Neural Stimuli

A control mechanism where action potentials in neurons stimulate the release of neurotransmitters directly onto endocrine cells to trigger hormone secretion.

Hormonal Stimuli

A control mechanism in which the secretion of a hormone is prompted or inhibited by the action of other hormones.

Tropic Hormone

A hormone whose primary function is to stimulate another endocrine gland to synthesize and secrete its own hormones.

Negative Feedback

A self-limiting control system where the accumulation of a hormone or its downstream biological effect inhibits its own further secretion once target levels are reached.

Positive Feedback

A self-propagating control system where a hormone promotes further synthesis and secretion of itself or its stimulating tropic hormone, amplifying the response.

Neurohypophysis

The posterior lobe of the pituitary gland, structurally continuous with the hypothalamus, which stores and secretes neurohormones synthesized in hypothalamic nuclei.

Adenohypophysis

The anterior lobe of the pituitary gland, composed of glandular epithelium, which synthesizes and secretes major hormones under the control of hypothalamic releasing and inhibiting factors.

Hypothalamohypophysial Portal System

A specialized capillary bed and vascular network that transports releasing and inhibiting hormones from the hypothalamus directly to the anterior pituitary gland.

Hypothalamohypophysial Tract

A bundle of nerve axons originating in the hypothalamus and terminating in the posterior pituitary, along which neurohormones are conducted for storage and release.

Antidiuretic Hormone (ADH)

Also known as vasopressin; a small peptide neurohormone stored in the posterior pituitary that promotes water reabsorption in the kidneys and constricts blood vessels to raise blood pressure.

Oxytocin

A small peptide neurohormone released by the posterior pituitary that stimulates uterine smooth muscle contractions during labor and induces milk ejection from mammary glands.

Growth Hormone (GH)

Also called somatotropin; an anterior pituitary protein that stimulates tissue growth, amino acid uptake, protein synthesis, lipid breakdown, and glycogen synthesis.

Gigantism

A clinical condition caused by chronic hypersecretion of growth hormone ($GH$) before the epiphyseal plates of long bones have ossified, leading to excessive linear height.

Acromegaly

A disorder resulting from elevated growth hormone ($GH$) secretion after the epiphyseal plates have ossified, characterized by thickening and widening of facial bones, hands, and feet without an increase in height.

Pituitary Dwarfism

A condition caused by insufficient secretion of growth hormone ($GH$) during childhood, resulting in delayed bone growth and short stature.

Prolactin (PRL)

A protein hormone secreted by the anterior pituitary that promotes milk production in the mammary glands of lactating women.

Thyroid-Stimulating Hormone (TSH)

Also called thyrotropin; a glycoprotein hormone secreted by the anterior pituitary that stimulates the synthesis and secretion of thyroid hormones from the thyroid gland.

Adrenocorticotropic Hormone (ACTH)

A peptide hormone from the anterior pituitary that stimulates the adrenal cortex to synthesize and secrete glucocorticoids, predominantly cortisol.

Thyroglobulin (Tg)

A large protein synthesized inside thyroid follicle cells that is exocytosed into the follicle lumen, serving as the iodinated protein scaffold for the formation of $T_3$ and $T_4$.

Triiodothyronine ($T_3$)

An iodinated thyroid hormone containing three iodine atoms that stimulates metabolic rate, protein synthesis, and oxygen consumption in almost all body tissues.

Tetraiodothyronine ($T_4$)

Also known as thyroxine; an iodinated hormone containing four iodine atoms produced by thyroid follicles that serves as the predominant circulating precursor to active $T_3$.

Calcitonin

A hormone produced by the parafollicular cells ($C$ cells) of the thyroid gland in response to high blood $Ca^{2+}$, acting to lower blood calcium by decreasing bone resorption.

Parathyroid Hormone (PTH)

A hormone released by the parathyroid glands in response to low blood $Ca^{2+}$ that raises blood calcium levels by stimulating osteoclasts and enhancing $Ca^{2+}$ reabsorption in kidneys and intestines.

Graves Disease

An autoimmune disorder in which thyroid-stimulating immunoglobulins (TSI) bind to and chronically activate TSH receptors, producing hyperthyroidism, goiter, and exophthalmos.

Exophthalmos

The abnormal forward protrusion or bulging of the eyes resulting from edema and connective tissue accumulation in retro-orbital spaces, commonly seen in Graves disease.

Myxedema

Non-pitting subcutaneous swelling of the face and extremities caused by mucoprotein deposits, typically found in individuals with severe hypothyroidism.

Goiter

An abnormal enlargement of the thyroid gland, which may occur in hyperthyroid states (such as Graves disease) or hypothyroid states resulting from iodine deficiency.

Adrenal Medulla

The inner core of the adrenal gland, derived from neural tissue, that synthesizes and secretes the catecholamines epinephrine and norepinephrine upon sympathetic stimulation.

Epinephrine

Also called adrenaline; a catecholamine hormone released by the adrenal medulla that increases heart rate, dilates skeletal muscle vessels, stimulates glycogenolysis, and elevates blood glucose during physical exertion or stress.

Zona Glomerulosa

The outermost histologic zone of the adrenal cortex, located beneath the connective tissue capsule, responsible for secreting mineralocorticoids such as aldosterone.

Aldosterone

The principal mineralocorticoid produced by the adrenal cortex that acts on the kidneys to promote sodium and water reabsorption, thereby raising blood pressure.

Zona Fasciculata

The intermediate, thickest layer of the adrenal cortex that synthesizes and secretes glucocorticoid hormones, primarily cortisol.

Cortisol

The primary glucocorticoid of the adrenal cortex that stimulates hepatic gluconeogenesis, enhances lipolysis and protein breakdown, and exerts potent anti-inflammatory and immunosuppressive effects.

Zona Reticularis

The innermost layer of the adrenal cortex bordering the medulla that secretes weak adrenal androgens such as androstenedione.

Pancreatic Acini

The exocrine clusters of cells in the pancreas that produce digestive enzymes and bicarbonate, discharging them through ducts into the duodenum.

Pancreatic Islets

Also known as the Islets of Langerhans; microscopic spherical clusters of endocrine cells within the pancreas containing alpha, beta, and delta cells.

Alpha ($\alpha$) Cells

Endocrine cells within pancreatic islets that synthesize and release glucagon when circulating glucose concentrations decline.

Beta ($\beta$) Cells

Endocrine cells within pancreatic islets that synthesize and release insulin when circulating glucose concentrations are elevated.

Delta ($\delta$) Cells

Endocrine cells within pancreatic islets that secrete somatostatin, an inhibitory hormone that suppresses both insulin and glucagon secretion locally.

Insulin

A peptide hormone produced by pancreatic beta cells that lowers blood glucose by facilitating glucose entry into target tissues and stimulating glycogen synthesis.

Glucagon

A peptide hormone produced by pancreatic alpha cells that elevates blood glucose by promoting hepatic glycogen breakdown (glycogenolysis) and gluconeogenesis.

Hemoglobin A1c (HbA1c)

A diagnostic blood test that measures the percentage of glycated hemoglobin in red blood cells to determine average blood glucose control over the preceding $3$ months.

Pineal Gland

A small endocrine gland located near the thalamus that responds to darkness and light via autonomic pathways, secreting melatonin and arginine vasotocin.

Melatonin

A hormone produced by the pineal gland during darkness that promotes sleep onset and inhibits the secretion of gonadotropin-releasing hormone ($GnRH$) from the hypothalamus.