Endocrine System and Hormones

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

The Endocrine System controls body functions through chemical messengers called hormones. Unlike the nervous system (fast, short-lived responses), the endocrine system produces slower, longer-lasting effects.

Key Differences: Nervous vs Endocrine System:

Feature	Nervous System	Endocrine System
Speed	Fast (milliseconds)	Slow (seconds to hours)
Effect duration	Brief	Long-lasting
Transmission	Electrical impulses	Blood-borne chemicals
Target	Specific organs	Wide-ranging effects

Hormones:

• Chemical nature: Proteins/peptides (e.g., insulin), steroids (e.g., testosterone, cortisol), amines (e.g., adrenaline)
• Action: Bind to specific receptors on target cells
• Feedback: Most hormone secretion is regulated by negative feedback

Major Endocrine Glands:

Gland	Hormone(s)	Main Function
Hypothalamus	Releasing/inhibiting hormones	Controls pituitary
Pituitary (anterior)	FSH, LH, TSH, ACTH, GH, Prolactin	Controls other glands, growth, milk production
Pituitary (posterior)	ADH, Oxytocin	Water balance, childbirth
Thyroid	T3, T4, Calcitonin	Metabolism, bone health
Parathyroid	PTH	Calcium balance
Pancreas	Insulin, Glucagon	Blood sugar regulation
Adrenal (cortex)	Cortisol, Aldosterone	Stress response, salt balance
Adrenal (medulla)	Adrenaline, Noradrenaline	Fight-or-flight response
Ovaries	Oestrogen, Progesterone	Female characteristics, menstruation
Testes	Testosterone	Male characteristics
Thymus	Thymosin	Immune system development
Pineal	Melatonin	Sleep-wake cycle

Adrenaline (Epinephrine):

• Released from adrenal medulla during stress
• Effects: Increased heart rate, blood pressure, pupil dilation, blood glucose rise, decreased digestion
• prepares body for “fight or flight”

⚡ WAEC Tip: The pancreas is BOTH exocrine (produces digestive enzymes released into duodenum) AND endocrine (produces insulin and glucagon released into blood). This is a common exam trick — don’t forget the dual function.

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🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding.

Mechanisms of Hormone Action:

1. Protein/Peptide Hormones (e.g., insulin, glucagon, GH):

• Cannot cross cell membrane (water-soluble)
• Bind to receptors on cell surface
• Activate second messenger systems inside cell
• Example: cAMP (cyclic AMP) — insulin uses PI3K pathway
• Faster response (seconds to minutes)

2. Steroid Hormones (e.g., cortisol, testosterone, oestrogen):

• Lipid-soluble (made from cholesterol)
• Can cross cell membrane
• Bind to intracellular receptors (in cytoplasm or nucleus)
• Act as transcription factors → affect gene expression
• Slower response (hours to days)

3. Amine Hormones (e.g., adrenaline, thyroid hormones):

• Adrenaline: Acts like protein hormones (cell surface receptors)
• Thyroid hormones: Act like steroid hormones (enter nucleus)

Control of Hormone Secretion — Negative Feedback:

Example: Thyroid hormone regulation:

• Hypothalamus secretes TRH (Thyrotropin Releasing Hormone)
• TRH stimulates anterior pituitary to release TSH (Thyroid Stimulating Hormone)
• TSH stimulates thyroid to release T3 and T4
• High T3/T4 levels INHIBIT both hypothalamus (TRH) and pituitary (TSH)

This is NEGATIVE feedback — high levels → reduced secretion

Other examples:

• High blood glucose → insulin release → glucose uptake → blood glucose falls → insulin secretion decreases
• High blood calcium → calcitonin release → calcium deposited in bone → blood calcium falls

Positive Feedback (less common):

• Oxytocin during childbirth: Contractions push baby → stretches cervix → signals hypothalamus → more oxytocin released → stronger contractions → cycle continues until birth
• LH surge at ovulation: Oestrogen rises → stimulates LH release → ovulation occurs

Key Hormones and Their Actions:

Growth Hormone (GH / Somatotropin):

• Stimulates protein synthesis and cell division
• Increases bone and muscle growth
• Promotes fat breakdown for energy
• Deficiency: Pituitary dwarfism (short stature, proportional)
• Excess (child): Gigantism (excessive height)
• Excess (adult): Acromegaly (enlarged hands, feet, jaw)

Insulin and Glucagon (Blood Sugar Regulation):

Feature	Insulin	Glucagon
Produced by	Beta cells (pancreas)	Alpha cells (pancreas)
Effect on blood glucose	Lowers	Raises
Stimulus	High blood glucose	Low blood glucose
Mechanism	Cells take up glucose; liver stores glucose as glycogen	Liver breaks down glycogen; releases glucose

Thyroid Hormones (T3 and T4):

• Increase basal metabolic rate
• Essential for normal growth and development
• Regulate protein synthesis
• Deficiency (childhood): Cretinism (mental retardation, stunted growth)
• Deficiency (adult): Myxoedema (fatigue, weight gain, cold intolerance)
• Excess: Hyperthyroidism (weight loss, rapid heart, heat intolerance)
• Goitre: Enlarged thyroid (iodine deficiency or hyperthyroidism)

Thyroid disorders are common in Nigeria due to iodine deficiency in inland areas (coastal fish provide iodine).

⚡ Common Student Mistakes: Confusing the roles of different glands. Confusing the effects of insulin and glucagon. Forgetting that negative feedback is what keeps hormone levels stable. Confusing endocrine with exocrine — endocrine glands secrete INTO blood; exocrine glands secrete OUT (e.g., sweat, digestive enzymes).

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🔴 Extended — Deep Study (3mo+)

Comprehensive theory for thorough preparation.

Hypothalamic-Pituitary Axis:

The hypothalamus is the “master coordinator” of the endocrine system:

Anterior Pituitary (controlled by hypothalamus via releasing/inhibiting hormones):

Hypothalamic Hormone	Pituitary Hormone	Target	Effect
TRH	TSH	Thyroid	T3/T4 secretion
CRH	ACTH	Adrenal cortex	Cortisol secretion
GnRH	FSH, LH	Ovaries/Testes	Gamete production, hormone secretion
GHRH	GH	Body tissues	Growth
Dopamine (PIH)	Prolactin	Mammary glands	Milk production
GnRH	—	—	(Inhibits prolactin)

Posterior Pituitary (stores and releases hypothalamic hormones):

• ADH (Antidiuretic Hormone / Vasopressin): Made in hypothalamus, stored in posterior pituitary
• Oxytocin: Made in hypothalamus, stored in posterior pituitary

Adrenal Glands:

Adrenal Cortex (produces steroid hormones):

1. Glucocorticoids (Cortisol):

   • Increases blood glucose (gluconeogenesis)
   • Anti-inflammatory (used as drugs: prednisolone, dexamethasone)
   • Released in response to stress (ACTH stimulation)
   • Chronic stress → sustained high cortisol → immunosuppression, muscle wasting

2. Mineralocorticoids (Aldosterone):

   • Increases Na⁺ reabsorption in kidneys
   • Water follows Na⁺ → blood volume increases → blood pressure rises
   • Regulated by renin-angiotensin-aldosterone system (RAAS)

3. Androgens (small amounts):

   • Converted to oestrogen in women
   • Contribute to libido and pubic hair development

Adrenal Medulla (produces catecholamines):

• Adrenaline (80%) and Noradrenaline (20%)
• Released during fight-or-flight response
• Effects mimic sympathetic nervous system but last longer
• Stress, exercise, low blood sugar all trigger release

Parathyroid Glands and Calcium Regulation:

Parathyroid Hormone (PTH):

• Increases blood calcium:
  • Bone resorption (osteoclasts break down bone)
  • Kidney reabsorbs calcium, excretes phosphate
  • Activates vitamin D → increases calcium absorption in intestine
• Low PTH (parathyroid damage during thyroid surgery): Hypocalcaemia → muscle tetany, seizures

Calcitonin (from thyroid C-cells):

• Decreases blood calcium (minor role in humans)
• More important in fish and amphibians

Reproductive Hormones:

Testosterone (Testes):

• Male sex characteristics (voice deepening, facial hair, muscle growth)
• Sperm production (spermatogenesis)
• Libido
• Produced by Leydig cells (stimulated by LH)

Oestrogen (Ovaries):

• Female sex characteristics (breast development, fat distribution)
• Thickens endometrium in menstrual cycle
• Inhibits FSH (negative feedback)
• Produced by granulosa cells of ovarian follicles

Progesterone (Ovaries):

• Maintains endometrium for implantation
• Inhibits FSH and LH (negative feedback)
• Produced by corpus luteum (after ovulation)
• Used in contraceptive pills

Melatonin (Pineal Gland):

• Regulates circadian rhythm (sleep-wake cycle)
• Secretion increases in darkness → drowsiness
• Light suppresses melatonin
• “Biological clock” — helps regulate other hormones

Thymosin (Thymus):

• Essential for T-lymphocyte development and maturation
• Children: Thymus is large; adults: Thymus shrinks
• Explains why children are more susceptible to infections

Hormonal Disorders:

Disorder	Cause	Hormone	Symptoms
Diabetes mellitus (Type 1)	Autoimmune destruction of beta cells	Low insulin	High blood glucose, glucose in urine
Diabetes mellitus (Type 2)	Insulin resistance	Low insulin or resistance	High blood glucose
Diabetes insipidus	Low ADH	Low ADH	Excessive dilute urine
Addison’s disease	Autoimmune destruction of adrenal cortex	Low cortisol, aldosterone	Weight loss, fatigue, low blood pressure
Cushing’s syndrome	Tumour or excess ACTH	High cortisol	Weight gain, high BP, muscle weakness
Acromegaly	Pituitary tumour (adult)	High GH	Enlarged hands, feet, jaw
Gigantism	Pituitary tumour (child)	High GH	Excessive height
Cretinism	Iodine deficiency (child)	Low thyroid hormone	Mental retardation, stunted growth

Endocrine Disruptors:

• Chemicals that interfere with hormone action
• Examples: BPA (bisphenol A), pesticides (DDT), PCBs
• Found in: Plastic containers, detergents, industrial chemicals
• Effects: Reproductive problems, developmental disorders, immune dysfunction
• Particularly concerning in Nigeria where pesticide use is increasing

Plant Hormones (Brief):

Hormone	Effect
Auxin	Cell elongation, phototropism, apical dominance
Gibberellin	Stem elongation, seed germination
Cytokinin	Cell division, delay senescence
Abscisic acid	Seed dormancy, stomatal closure
Ethylene	Fruit ripening, leaf abscission

⚡ WAEC Examination Patterns: Name the endocrine glands and their hormones. Explain how hormones work (protein vs steroid). Describe negative feedback with specific examples (thyroid, blood sugar). Explain the role of ADH in osmoregulation. Compare nervous and endocrine systems. Describe disorders like diabetes mellitus, goitre, and dwarfism.