INTRODUCTION TO ENDOCRINOLOGY - Complete Exam Summary (GK Pal Physiology)

1. BASIC DEFINITIONS

• Word "hormone" = Greek = "to excite"
• Secreted directly into general circulation (ductless glands)

2. ENDOCRINE GLANDS

A. Major Endocrine Glands (7)

1. Hypothalamus
2. Pituitary (anterior + posterior)
3. Thyroid
4. Adrenals (cortex + medulla)
5. Endocrine pancreas
6. Gonads (testes + ovary)
7. Parathyroid

B. Other Endocrine Organs

1. Thymus - secretes thymosin (childhood)
2. Kidney - secretes erythropoietin, renin
3. Heart - secretes ANP
4. GI tract - secretes many GI hormones
5. Placenta - secretes many hormones during pregnancy

3. FUNCTIONS OF HORMONES (9 points)

1. Change in cell function
2. Control of growth and development
3. Alteration in body mass and composition
4. Reproductive functions
5. Digestion, utilization and storage of nutrients
6. Regulation of volume and composition of fluid compartments
7. Behavioral changes
8. Control of nerve conduction
9. Act on target cells by binding to receptors → activates intracellular mechanisms → changes cell function

4. CLASSIFICATION OF HORMONES (Table 52.1)

A. Peptides/Proteins

• Insulin family: Insulin, IGF, relaxin
• Glycoproteins: LH, FSH, TSH, hCG
• Growth hormone family: GH, prolactin, placental lactogen
• Secretin family: Secretin, glucagon, GIP
• Others: ANP, calcitonin, CCK, ADH, somatostatin, ACTH, PTH

B. Amino Acid Derivatives

• Catecholamines: Dopamine, epinephrine, norepinephrine
• Iodinated amino acid: Thyroid hormones (T3, T4)

C. Steroids

• Glucocorticoids, mineralocorticoids, estrogen, progesterone, testosterone, 1,25-dihydroxycholecalciferol

5. SYNTHESIS OF HORMONES

Peptide Hormone Synthesis - 2 Pathways

Regulated Pathway:

• External stimuli trigger release of hormones already stored in secretory granules
• Example: GnRH from hypothalamus stimulates gonadotropin release from anterior pituitary
• Capable of releasing large amounts

Constitutive Pathway:

• Hormones secreted continuously from endocrine reticulum
• Formed from Golgi apparatus
• Promises secretory reserve; stimuli can also increase secretion

General Steps (Flowchart):

• Hormones also secreted from neoplastic tissues (called ectopic secretion / paraneoplastic syndrome)

Amine Hormone Synthesis:

• All except catecholamines → synthesized from enzymatic reactions involving amino acid tyrosine
• Serotonin - synthesized from 5-HT
• Catecholamines - synthesized from tyrosine; stored in calcium-mediated exocytosis

Steroid Hormone Synthesis:

• Synthesized from cholesterol; lipid soluble, hydrophilic
• NOT stored in cell granules - bound to proteins
• On stimulation → become free intracellular proteins → transported outside by diffusion

6. REGULATION OF HORMONE SECRETION (4 Mechanisms)

1. Feedback Control (most common)

• Negative Feedback - hormone concentration + metabolites inhibit the gland → maintains hormone homeostasis
  • Feedback may be simple or complex
  • Simple feedback: hormone from one gland directly inhibits secretion through feedback
  • Complex/Hierarchical feedback: 2nd or 3rd order feedback loops
    • 1st (upper) gland stimulates 2nd (middle) gland
    • 2nd gland stimulates 3rd (final/lower) gland
    • 3rd gland inhibits 1st or 2nd gland
    • Example: Hypothalamo-pituitary-thyroid axis, Hypothalamo-pituitary-adrenal axis, Hypothalamo-pituitary-gonadal axis
  • Long loop = target gland hormone inhibits hypothalamus/pituitary
  • Short loop = anterior pituitary hormone inhibits hypothalamus
  • Ultrashort loop = a hormone inhibits its own releasing hormone
• Positive Feedback - hormone concentration stimulates further secretion until peak → less common
  • Example: Estrogen surge before ovulation causes LH surge → Menstrual Cycle
  • Example: Developmental hormonal secretion at puberty

2. Neural Control

• Autonomic nervous system stimulates endocrine glands
• Stimulation of sympathetic/parasympathetic neurons → triggers hormone release (e.g., catecholamines from adrenal medulla)
• Receptor type in target tissue determines final secretion
• Secretion in response to stimuli (visual, olfactory, gustatory, etc.)

3. Rhythmic/Chronotropic Control

• Regulated by biological rhythms - sleep-wake cycle, seasonal rhythm, menstrual cycle
• Types:
  • Episodic: pulsatile, minutes to hours (e.g., LH)
  • Diurnal: cortisol peaks at day, melatonin at night
  • Developmental: at different phases of development (e.g., LH at puberty)
  • Seasonal: changes due to environmental changes
  • Periodic: gonadotropins at different phases of menstrual cycle

4. Humoral Control

• Control by hormones and chemicals (e.g., glucose stimulating insulin, angiotensin stimulating aldosterone)

7. HORMONE SIGNALING (3 Pathways)

8. INTERCELLULAR COMMUNICATIONS (5 mechanisms)

1. Electrical - gap junctions between cardiac myocytes
2. Neural - via synapses (synaptic transmission)
3. Endocrine, paracrine, autocrine - described in hormone signaling
4. Juxtacrine - via growth factors on cell surface
5. Neurocrine - acetylcholine to enrich parietal cells, non-cholinergic vagal fibers release GRP

9. TRANSPORT OF HORMONES

Peptide/Amino Acid Hormones:

• Readily dissolve in plasma - no special mechanism needed

Steroid and Thyroid Hormones:

• Insoluble in plasma - bound to carrier proteins (>90% bound)
• Thyroxine: TBG, TBPA, albumin
• Cortisol: Cortisol-binding globulin (CBG)
• Steroids/estrogen: Sex hormone binding globulin (SHBG)

• Protein-bound form = inactive pool of hormone
• Any condition altering carrier protein → alters hormone intensity

10. HORMONE DEGRADATION AND DISPOSAL

• Liver and kidney = major sites for extraction and degradation
• Urine = major excretion route for many hormones
• Bile = route for some hormones

Metabolic Clearance Rate (MCR):

• MCR is reported as mL plasma per min
• Shorter half-life → greater MCR

Estimation of Hormone Concentration:

1. RIA (Radioimmunoassay) - uses competitive binding principle; labels hormone with radioactive atom; measures immunoreactivity (not biological activity); major drawback - measures immunoreactivity, not biological activity
2. ELISA (Enzyme-linked Immunosorbent Assay) - enzyme-based colorimetric/fluorimetric assay; automated, large volume; superseded RIA in clinical use

11. HORMONE ACTIONS

Pleiotropic Effects:

• Hormones have several effects on target tissues (e.g., cortisol - not only gluconeogenesis but also influences lipolysis, protein catabolism, etc.)

Multiple Actions:

• Hormones have actions on different body systems

Complementary Actions:

• Short-term complementary: acute physical exercise - cortisol and glucagon both maintain blood glucose; deficiency = severe hypoglycemia
• Long-term complementary: growth regulated by GH, thyroid hormone, sex steroids

Antagonistic Actions:

• When one hormone opposes another on the same target
• Example: Insulin lowers glucose; glucagon raises glucose (liver)

12. KEY EXAM POINTS - QUICK RECALL

13. ANALOGY WITH NERVOUS SYSTEM

• Both are systems of communication
• Nervous system uses neurotransmitters at synaptic terminals
• Endocrine system uses hormones in blood
• Many neurons also secrete hormones → called neurohormones
• Immune cells (immunocytes) secrete cytokines - resemble hormones in function
• Some hormones act as neurotransmitters in the nervous system - called neurohormones

Exam Tip: Most common MCQs come from: hormone classification, feedback loops (long/short/ultrashort), synthesis steps (preprohormone → prohormone → hormone), transport (bound vs free), MCR definition, RIA vs ELISA, and types of signaling (endocrine/paracrine/autocrine/juxtacrine).