The endocrine system comprises glands located throughout the body that release specific chemical substances called hormones directly into the bloodstream. These hormones regulate varied functions of an organism, from bone growth to metabolism.
Understanding Hormones and Glands
Endocrine Glands
Endocrine glands secrete hormones directly into the bloodstream rather than into ducts. These chemical messengers travel through the blood to target organs and tissues, regulating essential bodily functions.
Exocrine Glands
Unlike endocrine glands, exocrine glands send their chemical substances into ducts that lead to the exterior of the body. Examples include sweat glands and salivary glands.
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Thyroid Gland Functions
Thyroxin & Triiodothyronine
These hormones increase metabolism in body cells, regulating energy production and consumption throughout the organism.
Calcitonin
This hormone lowers blood calcium levels, working in balance with parathyroid hormone to maintain proper calcium homeostasis.
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Parathyroid Glands
Parathyroid Hormone
The parathyroid glands produce parathyroid hormone, which increases blood calcium levels. This hormone works opposite to calcitonin, maintaining the delicate balance of calcium in the bloodstream essential for nerve function, muscle contraction, and bone health.
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Adrenal Glands: Cortex and Medulla
The adrenal glands sit atop the kidneys and consist of two distinct regions: the cortex (outer layer) and medulla (inner core). Each region produces different hormones with unique functions.
Adrenal Cortex
- Aldosterone increases sodium reabsorption
- Cortisol increases blood sugar levels
- Sex hormones maintain secondary characteristics
Adrenal Medulla
- Epinephrine (adrenaline) - sympathomimetic
- Norepinephrine (noradrenaline) - sympathomimetic
- Prepares body for fight-or-flight response
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Pancreatic Islet Cells
Insulin
Decreases blood sugar by converting glucose to glycogen for storage in the liver and muscles.
Glucagon
Increases blood sugar by converting stored glycogen back to glucose when energy is needed.
The pancreas plays a dual role as both an endocrine and exocrine gland. The islet cells function as the endocrine portion, regulating blood glucose levels through insulin and glucagon secretion.
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The Pituitary Gland: Anterior Lobe
The anterior pituitary produces multiple hormones that regulate growth, metabolism, and reproduction. Often called the "master gland," it controls many other endocrine glands.
Growth Hormone (GH)
Increases bone and tissue growth throughout the body.
TSH
Stimulates thyroxine production and thyroid gland growth.
ACTH
Stimulates hormone secretion from adrenal cortex, especially cortisol.
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Anterior Pituitary Gonadotropins
FSH
Follicle-stimulating hormone promotes oogenesis in females and spermatogenesis in males.
LH/ICSH
Luteinising hormone promotes ovulation in females and testosterone secretion in males.
Prolactin
Promotes breast tissue growth and milk secretion in nursing mothers.
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Pituitary Gland: Posterior Lobe
Antidiuretic Hormone (ADH)
Also called vasopressin, ADH stimulates reabsorption of water by kidney tubules, preventing excessive water loss and maintaining proper hydration.
Oxytocin
Stimulates contraction of the uterus during labour and childbirth, and also plays a role in milk ejection during breastfeeding.
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Ovarian Hormones
Oestradiol
Develops and maintains secondary sex characteristics in females, including breast development, body fat distribution, and regulation of the menstrual cycle.
Progesterone
Prepares and maintains the uterus during pregnancy, supporting the endometrial lining and preventing contractions during gestation.
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Testicular Hormones
Testosterone
The primary male sex hormone promotes growth and maintenance of secondary sex characteristics in males, including:
- Deepening of voice
- Facial and body hair growth
- Muscle mass development
- Bone density maintenance
- Sperm production
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Abnormal Endocrine Conditions
Endocrine disorders occur when glands produce too much or too little hormone, leading to various health complications. Understanding these conditions is essential for proper diagnosis and treatment.
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Thyroid Disorders
Hyperthyroidism
Overactivity of the thyroid gland resulting in excessive hormone production, increased metabolism, and symptoms like weight loss and rapid heartbeat.
Hypothyroidism
Underactivity of the thyroid gland causing insufficient hormone production, decreased metabolism, and symptoms like fatigue and weight gain.
Thyroid Carcinoma
Cancer of the thyroid gland requiring surgical intervention and potentially radioactive iodine treatment.
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Parathyroid Disorders
Hyperparathyroidism
Excessive production of parathormone leads to elevated blood calcium levels, potentially causing kidney stones, bone weakness, and cardiovascular issues.
Hypoparathyroidism
Deficient production of parathyroid hormone results in low blood calcium levels, causing muscle cramps, tingling sensations, and potential seizures.
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Adrenal Cortex Disorders
Adrenal Virilism
Excessive output of adrenal androgens causes masculinisation, including increased body hair, deepened voice, and altered body composition.
Cushing's Syndrome
A group of symptoms produced by excess cortisol from the adrenal cortex, including weight gain, moon face, buffalo hump, and skin changes.
Addison's Disease
Hypofunctioning of the adrenal cortex leads to insufficient cortisol and aldosterone production, causing fatigue, weight loss, and low blood pressure.
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Adrenal Medulla Disorder
Phaeochromocytoma
A tumour of the adrenal medulla where tumour cells stain a dark or dusky (phe/o) colour (chrom/o). This rare tumour causes excessive production of adrenaline and noradrenaline.
Symptoms include:
- Severe hypertension
- Rapid heartbeat
- Excessive sweating
- Headaches and anxiety
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Pancreatic Disorders
Hyperinsulinism
Excess secretion of insulin causes hypoglycaemia (low blood sugar), leading to shakiness, confusion, and potential loss of consciousness.
Diabetes Mellitus
Lack of insulin secretion or resistance to insulin prevents proper metabolism of sugar, starch, and fat in cells.
In diabetes mellitus, insulin insufficiency or ineffectiveness prevents sugar from leaving the blood (hyperglycaemia occurs) and entering body cells, where it is normally used to produce energy.
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Type I Diabetes Mellitus
Also called insulin-dependent diabetes mellitus (IDDM), Type I diabetes typically has onset in childhood and involves destruction of the beta islet cells of the pancreas with complete deficiency of insulin in the body.
Patient Profile
Patients are usually thin and younger, with sudden onset of symptoms including excessive thirst, frequent urination, and unexplained weight loss.
Treatment Requirements
Patients require frequent injections of insulin to maintain normal blood glucose levels. Careful monitoring and lifestyle management are essential.
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Type II Diabetes Mellitus
Also called non-insulin-dependent diabetes mellitus (NIDDM), Type II is a separate disease from Type I with a different inheritance pattern. Patients are usually older, and obesity is very common.
Pathophysiology
The islet cells are not destroyed. There is a relative deficiency of insulin secretion with resistance by target tissues to insulin action.
Treatment Approach
Treatment includes diet, weight reduction, exercise, and if necessary, insulin or oral hypoglycaemic agents.
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Primary Diabetes Complications
Ketoacidosis
Fats are improperly burnt, leading to accumulation of ketones in the body. This dangerous condition requires immediate medical attention.
Diabetic Coma
Occurs when blood sugar concentration gets too high or the patient receives insufficient insulin. Can be life-threatening without prompt treatment.
Hypoglycaemia
Can occur when too much insulin is taken by the patient, causing dangerously low blood sugar levels requiring immediate glucose administration.
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Secondary Diabetes Complications
Long-term complications occur over many years after a patient develops diabetes. These chronic conditions result from prolonged exposure to elevated blood glucose levels.
Diabetic Retinopathy
Destruction of blood vessels in the retina causes visual loss and blindness. Regular eye examinations are crucial for early detection.
Diabetic Nephropathy
Destruction of the kidneys causes renal insufficiency, often requiring haemodialysis or renal transplantation.
Atherosclerosis
Destruction of blood vessels leads to cardiovascular disease, increasing risk of heart attack and stroke.
Diabetic Neuropathy
Nerve destruction causes pain or loss of sensation, most commonly in the extremities, affecting quality of life.
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Pituitary Growth Disorders
Dwarfism
Congenital hyposecretion of growth hormone causes hypopituitary dwarfism, resulting in proportionate short stature.
Normal Growth
Appropriate growth hormone levels during childhood and adolescence result in normal height and development.
Gigantism
Hyperfunctioning of the pituitary gland before puberty leads to abnormal overgrowth of the body, resulting in excessive height.
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Acromegaly
Definition
Enlargement of the extremities (acro/o means extremities) caused by hypersecretion of growth hormone from the anterior pituitary after puberty.
Clinical Features
Patients develop enlarged hands, feet, and facial features including prominent jaw, enlarged nose, and thickened facial features. Internal organs may also enlarge.
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Additional Pituitary Disorders
Panhypopituitarism
All pituitary hormones are deficient, resulting in multiple endocrine deficiencies affecting growth, metabolism, reproduction, and stress response. Requires comprehensive hormone replacement therapy.
SIADH
Syndrome of inappropriate ADH involves excessive secretion of antidiuretic hormone, causing water retention, diluted blood sodium, and potential neurological complications.
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Diabetes Insipidus
Pathophysiology
Insufficient secretion of antidiuretic hormone (vasopressin) causes kidney tubules to fail to reabsorb needed water and salts.
Clinical Symptoms
Polyuria (excessive urination) and polydipsia (excessive thirst) are hallmark symptoms. Insipidus means tasteless, reflecting dilute urine.
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Laboratory Tests for Endocrine Function
Accurate diagnosis of endocrine disorders requires comprehensive laboratory testing. These tests measure hormone levels and metabolic markers to assess gland function.
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Blood and Urine Tests
Serum and Urine Tests
These tests measure hormones and other substances (electrolytes and glucose) in blood and urine as indicators of endocrine function.
Glucose Tolerance Test (GTT)
Measures glucose levels in fasting blood sample and in specimens taken 30 minutes, 1 hour, 2 hours, and 3 hours after ingesting 100 gm of glucose. Delayed return to normal indicates diabetes mellitus.
Radioimmunoassay (RIA)
Measures hormone levels in plasma based on antibodies' ability to bind specifically to radioactively labelled and non-radioactively labelled hormone molecules.
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Thyroid Function Tests
Thyroid Function Tests
Measure levels of T4, T3, and TSH in the bloodstream to assess thyroid gland function and diagnose hyper- or hypothyroidism.
Exophthalmometry
Measures the extent of eyeball protrusion as evidence in Graves' disease, an autoimmune form of hyperthyroidism.
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Thyroid Imaging Tests
Thyroid Scan
A radioactive compound is administered and localises in the thyroid gland. The gland is then visualised with a scanner device to detect tumours or nodules.
Radioactive Iodine Uptake
Radioactive iodine is administered orally, and its uptake into the thyroid gland is measured as evidence of thyroid function and metabolic activity.
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The Endocrine System