Questions
2 questions per theory paper
Difficulty
Medium
Importance
Foundational for physiology and clinical medicine
Overview
The endocrine system is a complex network of ductless glands that secrete hormones directly into the bloodstream to regulate physiological processes like metabolism, growth, and homeostasis. Understanding this topic is critical for exams as it bridges the gap between molecular signaling and systemic clinical pathologies. Mastery requires memorizing the feedback loops and the specific antagonistic actions between key hormonal pairs.
Pituitary Hormones
The pituitary gland, known as the master gland, is divided into the adenohypophysis and neurohypophysis, controlled by the hypothalamus. It secretes tropic hormones that regulate the activity of other endocrine glands throughout the body.
- Anterior lobe secretes GH, TSH, ACTH, FSH, LH, and Prolactin.
- Posterior lobe stores and releases Oxytocin and Vasopressin (ADH).
- Hypothalamic releasing hormones travel via hypophyseal portal system.
- GH stimulates IGF-1 release from the liver for bone/tissue growth.
- ADH regulates water reabsorption in renal distal convoluted tubules.
Thyroid Hormones
The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), which are essential for regulating the basal metabolic rate and protein synthesis. These hormones are unique as they contain iodine and exert widespread effects on almost every tissue in the body.
- T3 is the biologically active form; T4 is the major circulating form.
- Synthesis requires dietary iodine and thyroglobulin protein.
- Calcitonin acts to lower blood calcium levels by inhibiting osteoclasts.
- Regulated by TSH from the anterior pituitary via negative feedback.
- Hypothyroidism results in cretinism in children and myxedema in adults.
Adrenal Hormones
The adrenal glands consist of an outer cortex and an inner medulla, serving distinct roles in stress response and electrolyte balance. The cortex is stimulated by ACTH, while the medulla is controlled directly by the sympathetic nervous system.
- Cortex secretes Mineralocorticoids (Aldosterone) for Na+/K+ balance.
- Cortex secretes Glucocorticoids (Cortisol) to increase blood glucose.
- Cortex secretes Androgens as sex hormone precursors.
- Medulla produces Catecholamines (Epinephrine/Norepinephrine).
- Epinephrine triggers the 'fight or flight' response.
Pancreatic Hormones
The pancreas acts as both an exocrine and endocrine organ, with the Islets of Langerhans housing the endocrine cells. It maintains blood glucose homeostasis through the precise balance of insulin and glucagon.
- Beta cells secrete Insulin to decrease blood glucose levels.
- Alpha cells secrete Glucagon to increase blood glucose levels.
- Delta cells secrete Somatostatin to inhibit insulin and glucagon.
- Insulin facilitates glucose uptake into muscle and adipose tissue.
- Glucagon stimulates glycogenolysis and gluconeogenesis in the liver.
Exam Tip
Always draw a schematic feedback loop diagram when explaining hormone regulation, as examiners prioritize clear visual representation of regulatory axes over dense text.
Common Mistakes
- Confusing the action of TSH (thyroid-stimulating) with the thyroid hormones themselves.
- Misidentifying the source of hormone production, particularly mixing up the adrenal cortex and medulla.
- Ignoring the negative feedback loops which are essential for explaining regulation in long-form answers.
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