The Endocrine System

The Endocrine System and Hormones

Overview of the Endocrine System

• Definition: A collection of glands that produce hormones for regulating body functions.

• Function of Hormones: Chemical substances that have specific regulatory effects on organs, glands, or cell types.

Basic Concepts

Syndromes

• Definition: A group of symptoms, signs, laboratory results, and physiological disorders linked by common anatomical, biochemical, or pathological history.

Hormone Classifications

1. Polypeptide Hormones

   • Composed of amino acids linked by peptide bonds.

   • Water soluble and circulate freely in plasma as whole molecules or fragments.

   • Short half-life (<30 mins).

   • Bind to cell-membrane receptors to create a second messenger system.

2. Steroid Hormones

   • Derived from cholesterol with a four-ring structure.

   • Hydrophobic (water insoluble), primarily bound to carrier proteins.

   • Half-life ranges from 30-90 minutes.

3. Hormones Derived from Amino Acids

   • Derived from tyrosine (e.g., thyroxine, epinephrine) and tryptophan (e.g., serotonin).

   • Water soluble, circulate bound to proteins or free, with variable half-lives.

Key Hormones and Their Functions

Polypeptide Hormones Examples

• Adrenocorticotropin Hormone (ACTH): Stimulates adrenal gland activity.

• Antidiuretic Hormone (ADH): Regulates water balance.

• Insulin: Regulates glucose levels in the blood.

• Thyroid-Stimulating Hormone (TSH): Stimulates thyroid hormone production.

Steroid Hormones Examples

• Cortisol: Major glucocorticoid influencing metabolism and immune response.

• Estradiol (E2), Progesterone, Testosterone: Regulate reproductive functions.

Hormones Derived from Amino Acids

• Thyroxine (T4) and Triiodothyronine (T3): Critical in metabolism regulation.

• Epinephrine/Norepinephrine: Stress response hormones increasing heart rate and energy.

Types of Hormones

Tropic vs. Non-Tropic Hormones

• Tropic Hormones: Act on endocrine glands (e.g., TSH, ACTH).

• Non-Tropic Hormones: Directly act on target cells (e.g., cortisol).

Trophic Hormones

• Stimulate cellular growth and differentiation (e.g., GH).

Hormone Receptors

• Types: Membrane-bound (for protein hormones) and nuclear (for steroid hormones).

• Function: Binding to receptors triggers signal transduction mechanisms within the cell.

• First Messenger: The hormone; Second Messenger: Intracellular signaling molecules released after the first signal.

Feedback Mechanisms

Types of Feedback

1. Negative Feedback: Reduces hormone production when levels are sufficient.

2. Positive Feedback: Increases hormone production (less common in endocrine regulation).

Examples

• Cortisol regulation via CRH and ACTH.

• Thyroid hormone regulation via TRH and TSH.

Hormone Measurement Methods

Techniques

• Immunoassay: Measures hormone concentration using labeled antigens.

• Competitive: Inverse relation to hormone concentration.

• Immunometric: Directly proportional with two antibodies binding different sites of the hormone.

• Sandwich Immunoassay: Uses two antibodies for improved specificity.

Factors Influencing Hormone Concentration

• Biological Rhythms: Diurnal or circadian patterns.

• Protein Binding: Affects total hormone concentration.

• Variability: Hormones exist in different forms affecting their bioavailability.

Assessing Endocrine Function

• Evaluation of gland function/response (hyperfunctioning or hypofunctioning).

• Testing methods include assessing basal hormone levels, stimulation, and suppression tests.

Anterior Pituitary Hormones

Overview

• Functions: Secretion of hormones affecting growth, metabolism, and reproductive processes.

Key Anterior Pituitary Hormones

• ACTH: Stimulates cortisol production in adrenal cortex.

• FSH: Stimulates ovarian follicles in women and spermatogenesis in men.

• GH: Stimulates growth via IGF-1; imbalance leads to growth disorders.

• LH: Stimulates testosterone production in men and estrogen/progesterone in women.

Posterior Pituitary Hormones

ADH & Oxytocin

• ADH: Regulates fluid balance, involved in raising blood pressure.

• Oxytocin: Stimulates uterine contractions and milk ejection in lactation.

Adrenal Medullary Hormones

Catecholamines

• Epinephrine, Norepinephrine, Dopamine: Mediate fight-or-flight responses.

Pathologic Correlations

• Pheochromocytoma: Tumor causing catecholamine excess.

• Neuroblastoma: Malignancy causing catecholamine overproduction.

Adrenal Cortical Hormones

Glucocorticoids

• Cortisol: Influences metabolism, immune response, and stress adaptation.

Mineralocorticoids

• Aldosterone: Regulates sodium retention and potassium excretion.

Pathologic Conditions

• Addison’s Disease: Adrenal insufficiency with electrolyte imbalances.

• Cushing’s Syndrome: Abnormal cortisol elevations, often due to tumors.

Reproductive Hormones

Estrogens & Progesterone

• Estrogens: Induce secondary sexual characteristics and regulate menstrual cycle.

• Progesterone: Prepares the endometrium for implantation.

Androgens

• Testosterone & DHEA: Important for male characteristics and reproductive health.

Thyroid Hormones

Thyroxine (T4) and Triiodothyronine (T3)

• T4: Predominantly bound to transport proteins; indicator of thyroid function.

• T3: Active hormone involved in metabolic regulation.

Pathological Correlations

• Hypothyroidism: Symptoms include bradycardia, skin issues; often caused by autoimmune disorders

• Hyperthyroidism: Symptoms include weight loss, tachycardia; caused by conditions like Grave's Disease.