The endocrine system is a network of glands that produce hormones. These hormones act as chemical messengers and regulate many body functions such as growth, metabolism, reproduction, stress response, and homeostasis. Unlike the nervous system, endocrine responses are slower but have long-lasting effects.
Characteristics of Hormones
- Secreted directly into the bloodstream
- Act on specific target organs
- Effective in very small amounts
- Regulated by feedback mechanisms (mostly negative feedback)
Types of Hormones
- Peptide/Protein hormones: Insulin, growth hormone
- Steroid hormones: Cortisol, estrogen, progesterone
- Amino acid derivatives: Thyroxine, adrenaline
Major Endocrine Glands and Their Hormones
1. Hypothalamus
The hypothalamus controls the pituitary gland and links the nervous and endocrine systems.
- Corticotropin-releasing hormone (CRH)
- Thyrotropin-releasing hormone (TRH)
- Gonadotropin-releasing hormone (GnRH)
- Growth hormone-releasing hormone (GHRH)
2. Pituitary Gland
Also called the “master gland,” it secretes hormones that regulate many other endocrine glands.
Anterior Pituitary Hormones
- Growth hormone (GH)
- Prolactin
- Thyroid-stimulating hormone (TSH)
- Adrenocorticotropic hormone (ACTH)
- Follicle-stimulating hormone (FSH)
- Luteinizing hormone (LH)
Posterior Pituitary Hormones
- Oxytocin – involved in childbirth and milk ejection
- Antidiuretic hormone (ADH) – regulates water balance
3. Thyroid Gland
- Thyroxine (T4)
- Triiodothyronine (T3)
- Calcitonin – lowers blood calcium
4. Parathyroid Glands
- Parathyroid hormone (PTH) – increases blood calcium
5. Adrenal Glands
Cortex hormones
- Glucocorticoids – cortisol
- Mineralocorticoids – aldosterone
- Sex steroids – androgens
Medulla hormones
- Adrenaline (epinephrine)
- Noradrenaline
6. Pancreas (Islets of Langerhans)
- Insulin – lowers blood glucose
- Glucagon – raises blood glucose
- Somatostatin – inhibits GH and insulin
7. Gonads
Testes
- Testosterone
Ovaries
- Estrogen
- Progesterone
8. Other Endocrine Hormones
- Melatonin (pineal gland)
- Erythropoietin (kidneys)
- ANP (heart atria)
Endocrine Disorders (Overview)
1. Hypersecretion
- Gigantism (excess GH)
- Hyperthyroidism
- Cushing’s syndrome (high cortisol)
2. Hyposecretion
- Dwarfism (low GH)
- Hypothyroidism
- Addison’s disease (low cortisol)
Regulation of Hormone Secretion
Most hormones are controlled by a negative feedback mechanism where rising hormone levels inhibit further release.
Example: Low T3/T4 → hypothalamus releases TRH → pituitary releases TSH → thyroid releases hormones → hormone levels rise → TRH & TSH decrease.
Detailed Notes:
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