Endocrine Glands and Hormones
Endocrine System: Composed of ductless glands and diffused tissues/cells that secrete hormones. The neural and endocrine systems jointly coordinate and regulate physiological functions 2006
Hormone (Modern Definition): Non-nutrient chemicals which act as intercellular messengers and are produced in trace amounts.
Human Endocrine System
The Hypothalamus
- Basal part of the diencephalon (forebrain); regulates wide spectrum of body functions
- Contains neurosecretory cells (nuclei) that produce hormones
- Produces two types of hormones regulating anterior pituitary:
- Releasing Hormones: e.g., Gonadotrophin releasing hormone (GnRH) stimulates anterior pituitary to secrete LH and FSH NEET 2017
- Inhibiting Hormones: e.g., Somatostatin inhibits release of growth hormone from pituitary
Crucial detail: The posterior pituitary is under the direct neural regulation of the hypothalamus NEET-II 2016
The Pituitary Gland
Located in bony cavity called sella tursica and attached to hypothalamus by a stalk. Divided into Adenohypophysis (Pars distalis + Pars intermedia) and Neurohypophysis (Pars nervosa).
| Region | Hormones Secreted | Key Functions & Disorders |
|---|---|---|
| Pars distalis (Anterior Pituitary) |
Growth Hormone (GH) | Stimulates somatic growth. Over-secretion → Gigantism (children) 2011 or Acromegaly (adults, severe facial disfigurement) 2002, 2024. Low secretion → Pituitary dwarfism. Exception: Hypersecretion in adults does not increase height because epiphyseal plates close after adolescence NEET 2017 |
| Prolactin (PRL) | Regulates mammary gland growth and milk formation | |
| Thyroid Stimulating Hormone (TSH) | Stimulates synthesis and secretion of thyroid hormones | |
| Adrenocorticotrophic Hormone (ACTH) | Stimulates synthesis and secretion of glucocorticoids from adrenal cortex | |
| Luteinizing Hormone (LH) | Males: Stimulates Leydig cells to secrete androgens. Females: Induces ovulation of Graafian follicles and maintains corpus luteum | |
| Follicle Stimulating Hormone (FSH) | Males: Regulates spermatogenesis (along with androgens) 2023-Manipur. Females: Stimulates ovarian follicle development | |
| Pars intermedia | Melanocyte Stimulating Hormone (MSH) | Acts on melanocytes, regulates skin pigmentation 1988, 2000 |
| Pars nervosa (Posterior Pituitary) |
Oxytocin & Vasopressin | Crucial detail: Not a true endocrine gland because it only stores and releases hormones synthesized by hypothalamus NEET-II 2016, 2020-Covid |
| Oxytocin Vasopressin (ADH) |
Oxytocin: Stimulates smooth muscle contraction; causes vigorous uterine contraction during childbirth and milk ejection 1999, 2011, 2024 Re Vasopressin: Stimulates reabsorption of water/electrolytes by distal renal tubules, reducing urine water loss (prevents diuresis) 1991, 2011, 2020. Deficiency leads to Diabetes Insipidus 2004 |
The Pineal Gland
- Located on dorsal side of forebrain
- Secretes Melatonin, which critically regulates 24-hour (diurnal) rhythm of body (e.g., sleep-wake cycle, body temperature) 2000, 2011, 2014, 2023
- Also influences metabolism, pigmentation, menstrual cycle, and defense capabilities
Thyroid Gland
Composed of two lobes on either side of trachea, connected by connective tissue flap called isthmus. Composed of follicles and stromal tissues.
Crucial detail: It is the only endocrine gland that stores its hormone extracellularly before discharging it into blood 1995
Hormones & Functions
- Hormones: Tetraiodothyronine/Thyroxine (T₄) and Triiodothyronine (T₃)
- Functions: Regulates Basal Metabolic Rate (BMR) 1998, 2023, supports RBC formation 2023, controls metabolism of carbohydrates/proteins/fats, maintains water/electrolyte balance 2023
Disorders
- Hypothyroidism (Goitre): Enlargement of gland due to iodine deficiency 2010. During pregnancy, causes cretinism (stunted growth, mental retardation, deaf-mutism in baby) 2013, 2024. Note: Normal pregnancy naturally increases maternal thyroxine levels to support metabolic changes 2022 Re
- Hyperthyroidism: Abnormal high levels (e.g., due to cancer). Exophthalmic goitre (Graves' disease) characterized by enlarged thyroid, protruding eyeballs, increased BMR, weight loss NEET-II 2016, 2020, 2024
- Thyrocalcitonin (TCT): Protein hormone secreted by thyroid that lowers blood calcium levels 1998, 2011, 2023-Manipur
Parathyroid Gland
- Four glands present on back side of thyroid. Secretes peptide hormone Parathyroid Hormone (PTH)
- Secretion regulated by circulating levels of calcium ions (NOT sodium ions) 2023-Manipur
- Functions: It is a hypercalcemic hormone (increases blood Ca²⁺). Stimulates bone resorption (demineralisation) 2022, 2023-Manipur, reabsorption of Ca²⁺ by renal tubules, and absorption from digested food 2022
Crucial detail: PTH deficiency causes tetany 2003, 2004. Along with estrogen, plays significant role in osteoporosis NEET 2018
Thymus
- Lobular structure located between lungs behind sternum. Degenerates in old age, weakening immune responses
- Secretes peptide hormones called Thymosins
- Functions: Promotes differentiation of T-lymphocytes (provides cell-mediated immunity) and promotes antibody production (provides humoral immunity) 2012
Adrenal Gland
One pair, located above each kidney. Divided into outer cortex and inner medulla.
Adrenal Medulla
- Secretes Epinephrine (Adrenaline) and Norepinephrine (Noradrenaline), collectively called catecholamines
- Known as emergency hormones ("Fight or Flight")
- Functions: Increase heart beat, respiration rate, alertness, pupil dilation, and sweating 2011, 2013, 2014, 2024 Re. Stimulate glycogenolysis resulting in hyperglycemia 2024 Re, lipolysis, and proteolysis
Adrenal Cortex
Divided into zona reticularis (inner), zona fasciculata (middle), zona glomerulosa (outer). Secretes corticoids.
- Glucocorticoids (e.g., Cortisol): Stimulate gluconeogenesis 2020, lipolysis, and proteolysis. Produce anti-inflammatory reactions, suppress immune responses, stimulate RBC production. Hypersecretion causes Cushing's syndrome (moon face, hyperglycemia) 2024
- Mineralocorticoids (e.g., Aldosterone): Regulates water/electrolyte balance 1999. Acts on renal tubules to reabsorb Na⁺ and water, and excrete K⁺ and phosphate 2020
Disorder: Underproduction of corticoids causes Addison's disease (acute weakness, fatigue) 2020
Pancreas
A composite (exocrine + endocrine) gland. Endocrine part consists of Islets of Langerhans (1-2% of pancreatic tissue).
| Cell Type | Hormone | Action & Effect |
|---|---|---|
| α-cells | Glucagon (peptide) | Acts on hepatocytes to stimulate glycogenolysis and gluconeogenesis, resulting in hyperglycemia NEET-I 2016, 2020, 2024 Re |
| β-cells | Insulin (peptide) | Acts on hepatocytes and adipocytes 2020. Enhances cellular glucose uptake and glycogenesis, resulting in hypoglycemia 2013 |
Diabetes Mellitus: Caused by prolonged hyperglycemia (insulin deficiency/resistance). Characterized by glycosuria (glucose in urine) and ketonuria (ketone bodies in urine) 2004, 2020
Testis & Ovary
- Testis: Primary sex organ and endocrine gland. Leydig (interstitial) cells produce Androgens (Testosterone) 1993, 2024 Re. Regulates male accessory organs, spermatogenesis, libido, and has anabolic effects on protein/carbohydrates
- Ovary: Primary female sex organ. Produces Estrogen (synthesized by growing ovarian follicles) and Progesterone (secreted by corpus luteum post-ovulation) 1999, 2010
Crucial detail: Corpus luteum acts as a temporary endocrine gland NEET 2017
• Estrogen: Regulates secondary sex organs/characters
• Progesterone: Supports pregnancy and acts on mammary glands for alveoli formation
• Estrogen: Regulates secondary sex organs/characters
• Progesterone: Supports pregnancy and acts on mammary glands for alveoli formation
Hormones of Heart, Kidney, and GI Tract
| Organ | Hormone Produced | Primary Action |
|---|---|---|
| Heart (Atrial Wall) |
Atrial Natriuretic Factor (ANF) | Causes vasodilation, decreases blood pressure 2014, 2023-Manipur |
| Kidney (JG Cells) |
Erythropoietin | Stimulates erythropoiesis (formation of RBCs) 2021, 2024 Re |
| GI Tract | Gastrin | Stimulates gastric glands to secrete HCl and pepsinogen 1998, 2024 Re |
| Secretin | Acts on exocrine pancreas; stimulates secretion of water & bicarbonates 1998 | |
| Cholecystokinin (CCK) | Acts on pancreas & gall bladder; stimulates secretion of pancreatic enzymes & bile juice 1998, 2023-Manipur, 2024 Re | |
| Gastric Inhibitory Peptide (GIP) | Inhibits gastric secretion and motility |
Mechanism of Hormone Action
Hormones bind to specific proteins called hormone receptors in target tissues. Receptors are highly specific.
Chemical Classification of Hormones
- Peptide, Polypeptide, Protein: Insulin, glucagon, pituitary, and hypothalamic hormones 2012
- Steroids: Cortisol, testosterone, estradiol, progesterone 2006, 2024
- Iodothyronines: Thyroid hormones
- Amino-acid derivatives: Epinephrine. Crucial detail: Derived from amino acid tyrosine 1997, NEET 2018. Note: Melatonin and serotonin are derived from tryptophan NEET-I 2016
Modes of Action
Membrane-Bound Receptors
- Used by protein/peptide hormones (e.g., FSH)
- They do not enter the target cell. Instead, they bind to membrane and generate second messengers (like cAMP, IP₃, Ca²⁺) to regulate cellular metabolism 2012, 2022 Re
Note: Sodium is NOT a second messenger 2006
Intracellular (Nuclear) Receptors
- Used by steroid hormones and iodothyronines
- These hormones easily pass through the cell membrane 2012
- They bind to intracellular receptors to form a hormone-receptor complex that interacts directly with the genome to regulate gene expression or chromosome function NEET 2019