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Monday, August 11, 2025

Hormones


1. Definition and Overview

  • Hormones are chemical messengers secreted by specialized glands or tissues, transported through the bloodstream to target organs, where they regulate physiological and metabolic processes

  • They act at very low concentrations and bind to specific cellular receptors, initiating changes in gene expression or cell function

  • Classified broadly into endocrine hormones (secreted into the blood), paracrine hormones (acting locally), and autocrine hormones (acting on the same cell that secretes them)

  • Can be natural (produced by the body) or synthetic (used therapeutically to mimic or block natural hormone action)

2. Major Types and Chemical Classes

A. Peptide and Protein Hormones

  • Composed of amino acids; water-soluble; cannot cross cell membranes easily

  • Bind to membrane receptors and act through second messenger systems

  • Examples: insulin, glucagon, growth hormone, prolactin, parathyroid hormone

B. Steroid Hormones

  • Derived from cholesterol; lipid-soluble; easily cross cell membranes

  • Bind to intracellular receptors that regulate gene transcription

  • Examples: cortisol, aldosterone, estrogen, progesterone, testosterone

C. Amino Acid–Derived Hormones

  • Synthesized from tyrosine or tryptophan

  • Examples: thyroxine (T4), triiodothyronine (T3), epinephrine, norepinephrine, melatonin

D. Eicosanoids

  • Derived from arachidonic acid; act mainly in a paracrine or autocrine manner

  • Examples: prostaglandins, leukotrienes, thromboxanes

3. Sites of Hormone Production

  • Pituitary gland: Growth hormone, ACTH, TSH, LH, FSH, prolactin

  • Hypothalamus: Releasing and inhibiting hormones (e.g., TRH, CRH, GnRH, somatostatin)

  • Thyroid gland: T3, T4, calcitonin

  • Parathyroid glands: Parathyroid hormone (PTH)

  • Adrenal glands: Cortisol, aldosterone, adrenal androgens, catecholamines

  • Pancreas: Insulin, glucagon, somatostatin

  • Gonads: Estrogens, progesterone, testosterone, inhibin

  • Pineal gland: Melatonin

  • Other tissues: Kidney (erythropoietin, renin), heart (atrial natriuretic peptide), adipose tissue (leptin, adiponectin), GI tract hormones (gastrin, cholecystokinin, secretin, GLP-1)

4. Mechanisms of Action

  • Peptide hormones: Bind to surface receptors → activate second messenger cascades (e.g., cAMP, IP3/DAG) → alter enzyme activity and cellular metabolism

  • Steroid hormones: Diffuse into the cell → bind cytoplasmic/nuclear receptors → modify gene transcription → change protein synthesis

  • Thyroid hormones: Enter cells via transporters → bind nuclear receptors → regulate gene transcription

  • Catecholamines: Bind adrenergic receptors → activate G-protein coupled signaling pathways

5. Physiological Roles

  • Metabolism regulation: Insulin, glucagon, thyroid hormones, cortisol

  • Growth and development: Growth hormone, thyroid hormones, sex steroids

  • Reproduction: Estrogen, progesterone, testosterone, LH, FSH

  • Homeostasis: Aldosterone, ADH, PTH, calcitonin, ANP

  • Stress response: Cortisol, catecholamines

  • Immune modulation: Corticosteroids, thymic hormones

6. Therapeutic Uses of Hormones

  • Replacement therapy: Levothyroxine for hypothyroidism, insulin for diabetes mellitus, hydrocortisone for adrenal insufficiency

  • Contraception: Estrogen-progestin combinations, progestin-only formulations

  • Fertility treatment: Gonadotropins, clomiphene, GnRH analogs

  • Cancer treatment: Anti-estrogens, anti-androgens, aromatase inhibitors

  • Anti-inflammatory and immunosuppressive therapy: Corticosteroids

  • Growth disorders: Recombinant growth hormone for deficiency

  • Osteoporosis: Parathyroid hormone analogs, calcitonin

7. Adverse Effects of Hormonal Therapy

  • Corticosteroids: Immunosuppression, osteoporosis, hyperglycemia, Cushingoid appearance

  • Sex hormones: Thromboembolism, hypertension, increased cancer risk (estrogens), gynecomastia (androgens in men)

  • Thyroid hormones: Over-replacement can cause hyperthyroidism symptoms

  • Insulin: Hypoglycemia, weight gain

  • Growth hormone: Edema, arthralgia, insulin resistance

8. Regulation of Hormone Secretion

  • Negative feedback: Most common mechanism; elevated hormone levels inhibit further secretion (e.g., thyroid hormones inhibiting TSH release)

  • Positive feedback: Less common; hormone secretion stimulates further production (e.g., oxytocin during childbirth)

  • Neural control: Direct nervous system input (e.g., adrenal medulla catecholamine release via sympathetic activation)

  • Circadian rhythms: Hormone secretion varies by time of day (e.g., cortisol peaks in early morning)

9. Drug Interactions with Hormones

  • Enzyme inducers (e.g., rifampicin, phenytoin) increase metabolism of steroid hormones, reducing efficacy

  • Estrogens can enhance effects of corticosteroids

  • Thyroid hormone absorption reduced by calcium or iron supplements

  • Beta-blockers can mask symptoms of hypoglycemia in insulin-treated diabetics

10. Clinical Considerations

  • Careful monitoring of dosing to avoid under- or over-replacement

  • Long-term hormone therapy requires periodic evaluation for adverse effects

  • Synthetic analogs often have altered pharmacokinetics to improve efficacy or reduce side effects

  • Combination therapy may be needed in endocrine disorders involving multiple hormonal axes




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