PHA 329 - Endocrine System

Endocrine system

A system of ductless glands that secrete hormones into the bloodstream to regulate target tissues

Hormone

Chemical messenger released into the blood that binds receptors on target tissues to produce a physiological response

Endocrine gland

Group of epithelial-derived cells that secrete hormones directly into the bloodstream

Target tissue

Tissue that contains specific receptors for a hormone and responds when the hormone binds

Chemical classes of hormones

Peptide/protein hormones, steroid hormones, and amino acid–derived hormones

Peptide/protein hormones

Hormones made of amino acids; hydrophilic; stored in vesicles and released by exocytosis

Examples of peptide hormones

Insulin, growth hormone, TSH, oxytocin

Steroid hormones

Hormones synthesized from cholesterol; lipophilic; synthesized on demand and diffuse across membranes

Examples of steroid hormones

Cortisol, aldosterone, testosterone, estrogen, progesterone, vitamin D

Amino acid–derived hormones

Hormones derived from amino acids (usually tyrosine); can be hydrophilic or lipophilic

Examples of amino acid–derived hormones

Epinephrine, dopamine, thyroid hormone

Hydrophilic hormones

Hormones that are water-soluble, circulate freely in plasma, and bind cell membrane receptors

Lipophilic hormones

Hormones that are lipid-soluble, require carrier proteins in blood, and bind intracellular receptors

Protein hormone synthesis

Preprohormone → prohormone → active hormone with storage in secretory vesicles

Steroid hormone synthesis

Synthesized from cholesterol with pregnenolone as the rate-limiting step

On-demand synthesis

Hormone synthesis that occurs as needed rather than being stored, typical of steroid hormones

Hormone release via exocytosis

Method used by peptide and protein hormones to enter the bloodstream

Diffusion across membrane

Method used by lipophilic hormones to leave endocrine cells

Hormone transport

Movement of hormones through the bloodstream to target tissues

Carrier proteins

Plasma proteins that bind lipophilic hormones for transport in blood

Free hormone

Fraction of hormone not bound to carrier protein and biologically active

Cell membrane receptor

Receptor located on the plasma membrane used by hydrophilic hormones

Second messenger system

Intracellular signaling pathway activated by hydrophilic hormones after receptor binding

Intracellular receptor

Cytoplasmic or nuclear receptor used by lipophilic hormones

Gene transcription alteration

Process by which lipophilic hormone–receptor complexes change gene expression

Humoral stimulus

Hormone release triggered by changes in blood levels of ions or nutrients

Neural stimulus

Hormone release triggered by nervous system input

Hormonal stimulus

Hormone release triggered by another hormone (tropic hormone)

Negative feedback loop

Regulatory mechanism where the final hormone inhibits earlier hormone release to maintain homeostasis

Positive feedback loop

Regulatory mechanism where hormones amplify each other to produce a specific event

Homeostasis

Maintenance of a stable internal environment

Example of negative feedback

Thyroid hormone inhibiting TSH release

Example of positive feedback

Estrogen increasing LH and FSH during ovulation

Permissiveness

One hormone must be present for another hormone to exert its full effect

Example of permissiveness

Thyroid hormone required for normal reproductive development

Synergism

Two hormones produce the same effect and their combined effect is greater than additive

Example of synergism

Glucagon and epinephrine increasing blood glucose together

Antagonism

Two hormones have opposite physiological effects

Example of antagonism

Insulin lowers blood glucose while glucagon raises blood glucose

Up-regulation

Increase in number of receptors to enhance hormone signaling

Down-regulation

Decrease in number of receptors to reduce hormone signaling

Receptor desensitization

Decreased receptor responsiveness due to prolonged hormone exposure

Example of desensitization

Insulin resistance in type 2 diabetes

Hypothalamus

Neuroendocrine gland that produces releasing and inhibiting peptide hormones providing hormonal stimulation to the anterior pituitary and neural stimulation to the posterior pituitary

Posterior pituitary

Releases peptide hormones ADH (vasopressin) and oxytocin in response to neural stimulation from hypothalamic neurons

Anterior pituitary

Produces and releases peptide and protein hormones including GH (somatotropin), prolactin, TSH, ACTH, LH, and FSH in response to hormonal stimulation from hypothalamic releasing hormones

Antidiuretic hormone (ADH) / vasopressin

Peptide hormone released from the posterior pituitary via neural stimulation that increases renal water reabsorption, decreases urine output, causes vasoconstriction at high levels, and increases blood pressure

ADH release stimulation

Humoral stimuli including increased blood osmolarity, decreased blood pressure or volume, and hormonal stimulation by angiotensin II

ADH release inhibition

Humoral stimuli including low blood osmolarity, hypervolemia, and alcohol

Low ADH pathology

Diabetes insipidus characterized by excessive urination and dehydration

High ADH pathology

SIADH causing water retention, hyponatremia, and low urine output

Oxytocin

Peptide hormone released from the posterior pituitary via neural stimulation that stimulates uterine contractions during labor and milk ejection during breastfeeding

Oxytocin release stimulation

Neural stimulation from cervical stretch and breastfeeding with positive feedback

Oxytocin release inhibition

Stress

Growth hormone (GH) / somatotropin

Protein hormone released from the anterior pituitary via hormonal stimulation that promotes growth, protein synthesis, lipolysis, and increases blood glucose through IGF mediation

GH release stimulation

Hormonal stimulation by GHRH and humoral stimuli such as hypoglycemia, exercise, and deep sleep

GH release inhibition

Growth hormone inhibiting hormone (GHIH) also called somatostatin and negative feedback from GH and IGFs

Excess GH pathology

Gigantism in children and acromegaly in adults

GH deficiency pathology

Pituitary dwarfism in children

Growth hormone inhibiting hormone (GHIH) / somatostatin

Hypothalamic peptide hormone that inhibits growth hormone release from the anterior pituitary

Prolactin

Protein hormone released from the anterior pituitary that stimulates breast development and milk production and suppresses GnRH

Prolactin release stimulation

Hormonal stimulation by estrogen and neural stimulation from breastfeeding

Prolactin release inhibition

Prolactin inhibiting hormone (PIH) also called dopamine

Hyperprolactinemia

Elevated prolactin causing galactorrhea and infertility

Thyroid gland

Endocrine gland that produces amino acid derivative hormones T3 and T4 in response to hormonal stimulation by TSH

Triiodothyronine (T3)

Amino acid derivative thyroid hormone with three iodine atoms and high receptor affinity; primary physiologically active form

Thyroxine (T4) / tetraiodothyronine

Amino acid derivative thyroid hormone with four iodine atoms that serves as the primary circulating reservoir for conversion to T3

Thyroid peroxidase (TPO)

Enzyme responsible for iodide oxidation, iodination of tyrosine, and coupling reactions in thyroid hormone synthesis

Physiological effects of thyroid hormone

Increased basal metabolic rate, heart rate, respiration, GI motility, CNS activity, growth, and development

HPT axis

Hormonal stimulus pathway where TRH stimulates TSH release which stimulates thyroid hormone secretion with negative feedback

Hyperthyroidism

High thyroid hormone levels with low TSH

Hypothyroidism

Low thyroid hormone levels with high TSH

Adrenal cortex

Produces steroid hormones including mineralocorticoids, glucocorticoids, and adrenal androgens

Aldosterone

Steroid hormone classified as a mineralocorticoid released primarily by humoral stimuli that increases sodium and water reabsorption, potassium excretion, and blood pressure

Aldosterone release stimulation

Humoral stimuli including angiotensin II and hyperkalemia with minor hormonal stimulation by ACTH

Cortisol

Steroid hormone classified as a glucocorticoid released via hormonal stimulation that increases blood glucose, suppresses immune function, and mediates the stress response

HPA axis

Hormonal stimulus pathway where CRH stimulates ACTH release leading to cortisol secretion with negative feedback

Addison disease

Primary adrenal insufficiency characterized by low glucocorticoids and mineralocorticoids

Cushing syndrome

Condition of excess glucocorticoid cortisol causing hyperglycemia, immune suppression, and hypertension

Pancreatic beta cells

Endocrine cells that produce peptide hormones insulin and amylin

Insulin

Peptide hormone released in response to humoral stimulation that decreases blood glucose by increasing cellular glucose uptake and promoting energy storage

Insulin release stimulation

Humoral stimuli including hyperglycemia, amino acids, fatty acids, GLP-1, and acetylcholine

Insulin release inhibition

Epinephrine and norepinephrine

Type 1 diabetes

Autoimmune destruction of beta cells resulting in low insulin

Type 2 diabetes

Insulin resistance with initial hyperinsulinemia progressing to insulin deficiency

Pancreatic alpha cells

Endocrine cells that produce the peptide hormone glucagon

Glucagon

Peptide hormone released in response to humoral stimulation that increases blood glucose through glycogenolysis and gluconeogenesis

Glucagon release stimulation

Humoral stimuli including hypoglycemia, exercise, epinephrine, and norepinephrine

Oxytocin is produced in the and released from the .

Hypothalamus; posterior pituitary

Which of the following are physiological effects of antidiuretic hormone (ADH)? Select all that apply.

Vasoconstriction and renal water reabsorption

Which of the following are TRUE regarding growth hormone? Select all that apply.

Increases blood glucose; deep sleep increases its release

A patient with galactorrhea likely has which of the following hormone alterations?

Elevated prolactin

What is the difference in chemical composition between T3 and T4?

The number of iodine residues

A patient is diagnosed with an autoimmune condition that results in atrophy of the thyroid gland. Which abnormality would be expected?

Low T4 and high TSH

A patient is taking a glucocorticoid medication. Which of the following are expected effects? Select all that apply.

Increased blood glucose and immune suppression

Insulin release is primarily triggered by which of the following?

Increase in blood glucose

Amino acid–derived hormones

Hormones synthesized from amino acids including catecholamines and thyroid hormones

Examples of amino acid–derived hormones

Epinephrine, dopamine, and thyroid hormones (T3 and T4)

Catecholamines

Amino acid–derived hormones including epinephrine and dopamine that are water-soluble and act like peptide hormones

Epinephrine storage

Epinephrine is stored in secretory vesicles and released by exocytosis