Endocrine

Endocrinology Overview

  • Definition: Study of hormones and endocrine organs

  • Main Topics Covered:
        - Hormones
        - The pituitary gland & hypothalamus
        - The adrenal glands
        - The thyroid gland & parathyroid glands
        - The pancreas & other endocrine glands

Endocrine Glands

  • Characteristics:
        - Ductless glands
        - Secrete hormones directly into the bloodstream

  • Primary Functions:
        - Regulate body metabolism and energy balance
        - Facilitate growth and mobilization of body defenses
        - Aid in reproduction
        - Maintain electrolyte, water, and nutrient balance in blood

Major Endocrine Glands

  • Pituitary Gland

  • Thyroid Gland

  • Adrenal Gland

  • Pancreas

  • Ovary (f)

  • Testis (m)

  • Pineal Gland

  • Hypothalamus

  • Pancreatic Islet (of Langerhans)

Hormone Classification

  • Classes of Hormones:
        - Amines
            - Derived From: Tyrosine and tryptophan
            - Key Ideas: Small molecules
            - Examples: Hormones from adrenal medulla, thyroid, and pineal glands
            - Main Sources: Adrenal medulla, thyroid, pineal
        - Polypeptides & Proteins
            - Structure: Chains of amino acids (peptides/proteins)
            - Examples: ADH, insulin, GH
        - Glycopeptides
            - Structure: Long polypeptides bound to a carbohydrate
            - Examples: FSH, LH
        - Steroids
            - Derived From: Cholesterol
            - Key Ideas: Lipid-based hormones
            - Examples: Testosterone, estradiol, progesterone, cortisol

Biosynthetic Pathway for Steroid Hormones

  • Cholesterol → Pregnenolone → Progesterone
        - Secreted by Corpus Luteum of ovaries
        - In testes: Androstenedione
        - In adrenals: Cortisol
        - In ovaries: Estradiol-17

Hormonal Classifications by Action

  • Polar Hormones:
        - Water-soluble; cannot cross plasma membranes
        - Must be injected for drug application
        - Examples: Polypeptides, glycoproteins, catecholamines, norepinephrine, epinephrine

  • Nonpolar Hormones:
        - Lipophilic; can directly enter target cells
        - Can be taken orally
        - Examples: Steroids, thyroid hormone, melatonin

Prohormones vs Prehormones

  • Prohormones:
        - Inactive hormones needing cutting and splicing to become active
        - Example: Insulin

  • Prehormones:
        - Inactive prohormones needing modification within target cells

Hormone Interactions

  • Key Terms:
        - Permissiveness: One hormone's effect depends on another's presence
            - Example: Thyroid hormone + reproductive hormone for reproductive system development
        - Synergism: More than one hormone produces same effects
            - Example: Milk production requires estrogen, prolactin, and oxytocin
        - Antagonism: One hormone opposes another
            - Example: Insulin promotes fat storage, glucagon promotes fat breakdown

Control of Hormone Release

  • Mechanisms:
        - Controlled by negative feedback systems
        - Stimulated by:
            - Humoral stimuli (e.g., low calcium levels stimulating PTH)
            - Neural stimuli (e.g., sympathetic input to adrenal medulla)
            - Hormonal stimuli (e.g., hypothalamic releasing hormones)

Mechanisms of Action

  • General Binding Characteristics:
        - Specific and high-affinity binding to receptors
        - Low receptor capacity, saturation possible

  • Receptor Types:
        - Lipophilic Hormone Receptors: Located in cytoplasm or nucleus
        - Water-Soluble Hormone Receptors: Located on plasma membrane

Action Mechanisms of Lipophilic Hormones

  • Travel attached to carrier proteins, dissociating at target cells

  • Bind to nuclear hormone receptors, activating genetic transcription resulting in protein production

  • Example: Thyroxine (T4) is converted to T3 inside the target cell; receptors bound to DNA in nucleus

Action Mechanisms of Water-Soluble Hormones

  • 2nd Messenger Mechanisms:
        - Activate intracellular mediators via surface receptors
        - Include:
            - Adenylate Cyclase
            - Phospholipase C
            - Tyrosine Kinase

Adenylate Cyclase (cAMP) System

  • Process:
        - Hormones like epinephrine bind to β-adrenergic receptors
        - G-protein activation → adenylate cyclase → ATP to cAMP → activates protein kinase → alters metabolism

Phospholipase C System

  • Process:
        - Hormones bind to α-adrenergic receptors
        - G-protein activation → phospholipase C → production of IP3/DAG → Ca2+ liberation → calmodulin/kinases alter enzymes

Pituitary Gland Overview

  • Structure: Attached to hypothalamus via infundibulum; composed of anterior (adenohypophysis) and posterior (neurohypophysis) lobes

  • Function of Posterior Lobe:
        - Stores and releases ADH and oxytocin produced by hypothalamus

Anterior Lobe Hormones

  • Hypothalamic Function: Hormones travel through portal veins to anterior pituitary, stimulating/inhibiting hormone release
        - Main Hormones Released: GH, TSH, ACTH, FSH, LH, PRL

Adrenal Glands Structure

  • Located atop kidneys; divided into adrenal cortex and adrenal medulla
        - Functions of Medulla: Secretes epinephrine/norepinephrine in response to sympathetic stimulation
        - Functions of Cortex: Secretes corticoseroids (mineralocorticoids, glucocorticoids, gonadocorticoids)

Hormone Functions of Adrenal Cortex

  • Mineralocorticoids: Regulation of Na+ and K+ balance (e.g., aldosterone)

  • Glucocorticoids: Control of glucose metabolism (e.g., cortisol)

  • Gonadocorticoids: Weak sex hormones contributing to secondary sexual characteristics

Thyroid Gland Overview

  • Location: Below larynx with follicles producing hormones (thyroxine & calcitonin)

  • Thyroid Hormone Production: Uses iodine, attaching it to thyroglobulin, leading to T3 and T4 production

Disorders of Thyroid Gland

  • Iodine Deficiency: Can lead to goiter, hypothyroidism, and growth-related issues

  • Grave’s Disease: Hyperthyroidism and signs like exophthalmos (bulging eyes)

Parathyroid Hormones

  • Function: PTH increases blood calcium by acting on bones, kidneys, intestines; counteracts effects of calcitonin

  • Dysfunctions: Can lead to calcium imbalances and associated ailments

Pancreas Hormones

  • Endocrine Function: Insulin (hypoglycemic hormone) regulates blood glucose levels

  • Glucagon: After fasting, raises blood glucose levels by promoting glycogenolysis and gluconeogenesis

Pineal Gland Function

  • Produces Melatonin, regulating circadian rhythms and seasonal functions

Other Endocrine Structures

  • Heart: Secretes ANP to regulate blood pressure

  • Kidneys: Produce erythropoietin for RBC production and renin for blood pressure regulation

Prostaglandins and Inhibitors

  • Prostaglandins: Derived from arachidonic acid; involved in inflammation and pain responses

  • NSAIDs: Inhibit COX enzymes to reduce prostaglandin levels, impacting pain and inflammation

Case Studies Evaluation

  • Multiple scenarios for assessing understanding of hormonal effects, interactions, and therapeutic considerations in clinical practice (e.g., diabetes management, stress impact, thyroid dysfunction).