Introduction to Hormones
🧪 Hormones are chemical messengers produced by glands in the endocrine system. They regulate growth, metabolism, reproduction, mood, and immune function.
Secreted directly into the bloodstream, they travel to target organs to ensure coordinated body function. ⚖️
📖 About
- Hormones are secreted by endocrine glands (e.g. pituitary, thyroid, adrenals, pancreas).
- They act on distant target tissues, regulating vital processes like growth, reproduction, energy balance, and stress response.
- They can be classified by structure (steroid, peptide, amino acid–derived, glycoproteins, eicosanoids) or function (metabolic, reproductive, stress-related).
🧬 Types of Hormones
- 💊 Steroid Hormones (lipid-soluble, cholesterol-derived): cortisol, aldosterone, testosterone, oestrogen.
- Mechanism: Cross lipid bilayer → bind intracellular receptor → hormone–receptor complex binds DNA → alters gene transcription. 🧬
- Effects: Long-lasting (hours–days), regulate reproduction, metabolism, inflammation, and fluid balance.
- Examples: Cortisol (stress, metabolism), Aldosterone (salt balance), Testosterone/oestrogen (sexual development).
- 🧩 Peptide Hormones (short amino acid chains, water-soluble): insulin, glucagon, GH, ADH.
- Mechanism: Bind cell-surface receptors → activate second messengers (cAMP, IP₃) → rapid cellular effects. ⚡
- Effects: Fast but short-lived.
- Examples: Insulin (↓ blood glucose), Glucagon (↑ blood glucose), GH (growth).
- 🔗 Amino Acid–Derived Hormones: small molecules from tyrosine/tryptophan.
- Examples:
- Thyroxine (T₄) & Triiodothyronine (T₃): regulate metabolism 🌀
- Epinephrine/Norepinephrine: fight-or-flight response ⚔️
- Melatonin: sleep–wake cycle 🌙
- Can be lipid-soluble (thyroid hormones) or water-soluble (catecholamines).
- 🔥 Eicosanoids (fatty acid–derived): prostaglandins, thromboxanes, leukotrienes.
- Local action (paracrine/autocrine): inflammation, pain, clotting, immunity.
- Clinical note: NSAIDs (e.g. ibuprofen) block prostaglandin synthesis → ↓ pain and fever.
- 🧷 Glycoprotein Hormones: peptide hormones with sugar chains (FSH, LH, TSH).
- Mechanism: Bind cell-surface receptors → second messengers.
- Examples: FSH (spermatogenesis/follicle development), LH (ovulation/testosterone release), TSH (stimulates thyroid).
📊 Diagram
⚖️ Hormonal Regulation
- Most regulation is via negative feedback (like a thermostat). ❄️🔥
- Example: TSH–T3/T4 axis → rising T3/T4 inhibits TRH & TSH.
- Example: Insulin–glucagon balance → maintains glucose homeostasis.
- Positive feedback is rare but important:
- Oxytocin in childbirth → stronger contractions 👶
- Platelet aggregation → clot formation 🩸
- Circadian rhythms: Cortisol (morning peak 🌅), Melatonin (night rise 🌙).
- Receptor regulation:
- Downregulation: prolonged high hormone → ↓ receptors (e.g. insulin resistance in type 2 diabetes).
- Upregulation: low hormone → ↑ receptors (e.g. ↑ oxytocin receptors in pregnancy).
🏛️ Hypothalamic–Pituitary Control
- HPA axis: CRH → ACTH → cortisol (stress response). 😰
- HPG axis: GnRH → FSH/LH → sex hormones (reproduction). 💕
- HPT axis: TRH → TSH → T3/T4 (metabolism). 🔥
🔑 Key Hormones by Gland
- 🧠 Anterior Pituitary: GH, TSH, ACTH, FSH, LH, Prolactin.
- 🧠 Posterior Pituitary: ADH (water balance), Oxytocin (labour, lactation).
- 🦋 Thyroid: T3, T4 (metabolism), Calcitonin (↓ calcium).
- 🦴 Parathyroid: PTH (↑ calcium, bone resorption).
- 🧷 Adrenal Cortex:
- Mineralocorticoids (aldosterone: salt & water balance)
- Glucocorticoids (cortisol: stress, metabolism)
- Androgens (minor sex hormones)
- ⚡ Adrenal Medulla: Epinephrine, Norepinephrine, Dopamine → fight-or-flight.
📌 Conclusion
Hormones act as the body’s “messenger system,” regulating nearly every physiological process.
Disruption of these pathways → endocrine disorders (e.g. diabetes, hypothyroidism, Cushing’s).
🧬 Understanding hormone regulation helps in both diagnostics and therapy, from insulin use in diabetes to thyroid hormone replacement.
