Quick Summary
The endocrine system uses chemical messengers (hormones) for slow but long-lasting coordination. Hormones act on specific target cells via receptors. This chapter covers all major endocrine glands, their hormones, functions, and disorders. Endocrinology contributes 4–5 questions to NEET biology every year.
Overview of Endocrine Glands
| Gland | Location | Key Hormones |
|---|---|---|
| Hypothalamus | Brain | Releasing/inhibiting hormones (control pituitary) |
| Pituitary | Base of brain (sella turcica) | GH, TSH, ACTH, FSH, LH, Prolactin, ADH, Oxytocin |
| Thyroid | Neck | T3, T4, Calcitonin |
| Parathyroid | Behind thyroid (4 glands) | PTH (parathormone) |
| Adrenal | Above kidneys | Adrenaline, Noradrenaline, Cortisol, Aldosterone |
| Pancreas | Behind stomach | Insulin, Glucagon |
| Gonads | Testes/Ovaries | Testosterone, Oestrogen, Progesterone |
| Thymus | Chest (childhood) | Thymosin (T-cell maturation) |
| Pineal | Brain | Melatonin (sleep cycle, circadian rhythm) |
Pituitary Gland — "Master Gland"
Anterior pituitary (adenohypophysis):
| Hormone | Target | Effect |
|---|---|---|
| GH (somatotropin) | Bones, muscles | Growth; excess → gigantism/acromegaly; deficiency → dwarfism |
| TSH | Thyroid | Stimulates T3/T4 secretion |
| ACTH | Adrenal cortex | Stimulates cortisol secretion |
| FSH | Gonads | Follicle development (female), sperm production (male) |
| LH | Gonads | Ovulation (female), testosterone (male) |
| Prolactin | Mammary glands | Milk production |
| MSH | Melanocytes | Skin pigmentation |
Posterior pituitary (neurohypophysis): Stores and releases hypothalamic hormones:
- ADH (vasopressin): Water reabsorption in DCT/collecting duct
- Oxytocin: Uterine contractions (labour), milk ejection
Thyroid Gland
- T3 (triiodothyronine) and T4 (thyroxine):
- Regulate BMR (basal metabolic rate)
- Essential for normal growth and brain development
- Require iodine for synthesis
- Deficiency: Hypothyroidism (in adults = myxoedema; in infants = cretinism)
- Excess: Hyperthyroidism (Graves' disease — exophthalmos)
- Iodine deficiency → Goitre (swelling of thyroid)
- Calcitonin: Lowers blood Ca²⁺ (promotes bone deposition; opposes PTH)
Parathyroid Gland
- PTH (parathormone): Raises blood Ca²⁺
- Promotes bone resorption
- Increases Ca²⁺ reabsorption in kidney
- Activates Vitamin D → more intestinal Ca²⁺ absorption
Adrenal Gland
Adrenal Cortex (steroid hormones)
| Hormone | Effect |
|---|---|
| Glucocorticoids (cortisol) | Stress response; anti-inflammatory; gluconeogenesis; excess → Cushing's syndrome |
| Mineralocorticoids (aldosterone) | Na⁺ reabsorption (↑ blood volume, ↑ BP); deficiency → Addison's disease |
| Androgens | Sex hormones (minor; important in females) |
Adrenal Medulla (catecholamines)
- Adrenaline (epinephrine) and Noradrenaline: Fight-or-flight response
- ↑ heart rate, ↑ blood pressure, ↑ glucose, bronchodilation, pupil dilation
Pancreas — Islets of Langerhans
| Cell | Hormone | Effect |
|---|---|---|
| α cells | Glucagon | ↑ blood glucose (glycogenolysis, gluconeogenesis) |
| β cells | Insulin | ↓ blood glucose (glucose uptake by cells, glycogenesis) |
| δ cells | Somatostatin | Inhibits both insulin and glucagon |
Diabetes mellitus:
- Type 1 (IDDM): Autoimmune destruction of β cells → no insulin
- Type 2 (NIDDM): Insulin resistance → inadequate response
Gonads
| Gland | Hormone | Function |
|---|---|---|
| Testes | Testosterone | Secondary male characters, spermatogenesis |
| Ovaries | Oestrogen | Secondary female characters, uterine lining development |
| Ovaries | Progesterone | Maintains pregnancy, prepares uterus |
Feedback Regulation
Most hormones are regulated by negative feedback: High T4 → inhibits TSH release → T4 decreases → TSH increases again (Hypothalamus → TRH → Pituitary → TSH → Thyroid → T4)
Important Questions
One Mark
- Name the hormone that regulates BMR.
- Which hormone is called the "emergency hormone"?
- Name the cells in the islets of Langerhans that secrete insulin.
- What is the function of melatonin?
Short Answer (3 marks)
- Compare diabetes mellitus and diabetes insipidus.
- Name the hormones of the adrenal cortex and state one function each.
- What are the effects of excess and deficiency of GH?
Long Answer (5 marks)
- Describe the hormones secreted by the anterior pituitary and their functions.
- Explain the role of the pancreatic hormones in blood glucose regulation.
Common Mistakes
- Posterior pituitary: does NOT synthesise ADH and oxytocin — it only stores and releases them (synthesised in hypothalamus).
- Insulin LOWERS blood glucose; glucagon RAISES it — students sometimes reverse these.
- Calcitonin (thyroid) lowers Ca²⁺; PTH (parathyroid) raises Ca²⁺ — opposite effects.
- Addison's disease: deficiency of adrenal cortex hormones (cortisol + aldosterone). Cushing's syndrome: excess cortisol.
Board Exam Tips
- Anterior pituitary hormones table (7 hormones) is a standard 5-mark question.
- "Compare diabetes mellitus and diabetes insipidus" appears as a 3-mark question every year.
- Adrenal gland — cortex vs medulla hormones and functions is a reliable 3-mark question.
- NEET: Pituitary hormones, thyroid disorders, insulin/glucagon, and feedback regulation contribute 4–5 questions per paper.
Frequently Asked Questions
GH (growth hormone), TSH (thyroid-stimulating), ACTH (adrenocorticotropic), FSH and LH (gonadotropins), Prolactin (milk production), MSH (melanocyte-stimulating). The anterior pituitary is controlled by hypothalamic releasing and inhibiting hormones.