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DrX Whiz Niraj
Endocrine System: Hormones & Glands
Master the master glands! Complete exam notes on Hormone classification, Mechanism of action, Endocrine organs, and Clinical disorders.
The Endocrine System consists of all endocrine glands and hormone-secreting cells. Unlike exocrine glands (which secrete products into ducts, e.g., sweat glands), endocrine glands are ductless and secrete their products (Hormones) directly into the interstitial fluid, which then diffuses into the bloodstream.
Chemical Classification of Hormones:
Hormones are chemically classified into two broad classes based on their solubility. This solubility determines how they act on target cells.
- 1. Lipid-Soluble Hormones: Can cross the lipid bilayer of the cell membrane.
Examples: Steroid hormones (Cortisol, Testosterone, Estrogen), Thyroid hormones (T3, T4), Nitric oxide (NO). - 2. Water-Soluble Hormones: Cannot cross the cell membrane. They circulate freely in the blood.
Examples: Amine hormones (Adrenaline, Histamine), Peptide/Protein hormones (Insulin, Oxytocin, ADH), Eicosanoids.
Hormones only affect specific Target Cells that possess specific protein receptors for that hormone. The mechanism differs entirely for lipid-soluble vs water-soluble hormones.
A. Lipid-Soluble Mechanism
Because they are lipid-soluble, these hormones easily diffuse through the plasma membrane.
Their receptors are located Inside the cell (in the cytoplasm or nucleus). The hormone-receptor complex alters gene expression (DNA transcription), forming new proteins that alter the cell's activity.
B. Water-Soluble Mechanism
They cannot cross the lipid membrane. They bind to Receptors on the outer surface of the plasma membrane (acting as the first messenger).
This binding activates an enzyme (Adenylate cyclase) which converts ATP into Cyclic AMP (cAMP). cAMP acts as the second messenger, activating enzymes inside the cell to cause the physiological response.
For many years, the Pituitary gland (Hypophysis) was called the "master endocrine gland" because it secretes several hormones that control other endocrine glands. However, we now know that the pituitary itself is controlled by the Hypothalamus (the major integrating link between the nervous and endocrine systems).
| Lobe of Pituitary | Hormones Secreted | Main Function / Target |
|---|---|---|
| Anterior Pituitary (Adenohypophysis) |
1. hGH (Human Growth Hormone) 2. TSH (Thyroid-stimulating hormone) 3. ACTH (Adrenocorticotropic hormone) 4. FSH & LH (Gonadotropins) 5. Prolactin (PRL) |
1. Growth of bones/muscles 2. Stimulates Thyroid gland 3. Stimulates Adrenal cortex 4. Stimulates Ovaries/Testes 5. Milk production in females |
| Posterior Pituitary (Neurohypophysis) |
1. Oxytocin (OT) 2. Antidiuretic Hormone (ADH / Vasopressin) |
(Note: It does not synthesize these, only stores them from hypothalamus). 1. Uterus contraction during labor & milk ejection. 2. Decreases urine volume (saves water). |
Thyroid & Parathyroid
- Thyroid Gland: Secretes Thyroxine (T4) and Triiodothyronine (T3). These regulate basal metabolic rate (BMR). Requires Iodine for synthesis. It also secretes Calcitonin (lowers blood calcium).
- Parathyroid Gland: Secretes Parathyroid Hormone (PTH). It increases blood calcium levels (antagonist to calcitonin).
Adrenal Glands
Located on top of each kidney. Has two parts:
- Adrenal Cortex (Outer): Secretes steroid hormones. Aldosterone (mineralocorticoid for Na+/water retention), Cortisol (glucocorticoid for stress/inflammation), and Androgens.
- Adrenal Medulla (Inner): Secretes Adrenaline (Epinephrine) and Noradrenaline. Enhances the fight-or-flight response.
Pancreas
It is both an exocrine and an endocrine gland. The endocrine tissue is called Islets of Langerhans.
- Alpha cells: Secrete Glucagon (Raises blood glucose).
- Beta cells: Secrete Insulin (Lowers blood glucose).
Pineal & Thymus
- Pineal Gland: Located in the brain. Secretes Melatonin, which regulates the body's biological clock (sleep-wake cycle).
- Thymus: Located behind the sternum. Secretes Thymosin, which promotes the maturation of T-cells (Immunity). It shrinks as we age.
Clinical Correlation (Applied Pathophysiology)
Understanding these disorders is critical for Pharmacology:
- Pituitary Gland: Dwarfism (Hyposecretion of hGH in childhood), Gigantism (Hypersecretion of hGH in childhood), Acromegaly (Hypersecretion of hGH in adults; thick jaw/hands).
- Thyroid Gland: Goiter (Enlarged thyroid due to Iodine deficiency), Graves' Disease (Autoimmune hyperthyroidism, bulging eyes).
- Adrenal Gland: Cushing's Syndrome (Hypersecretion of Cortisol; moon face, buffalo hump), Addison's Disease (Hyposecretion of glucocorticoids/aldosterone).
- Pancreas: Diabetes Mellitus. Type 1 (Beta cells destroyed, absolute lack of insulin). Type 2 (Insulin resistance by target cells).