Endocrine System

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Endocrine System — Key Facts The endocrine system consists of ductless glands that secrete hormones directly into the bloodstream. These chemical messengers regulate metabolism, growth, reproduction, and stress responses.

Major Endocrine Glands (Human)

Gland	Primary Hormone(s)	Function
Hypothalamus	Releasing/inhibiting hormones	Controls pituitary
Pituitary (Anterior)	TSH, ACTH, FSH, LH, GH	Growth, reproduction, metabolism
Pituitary (Posterior)	ADH, Oxytocin	Water retention, uterine contraction
Thyroid	T3, T4, Calcitonin	Basal metabolic rate, calcium
Parathyroid	PTH	Calcium homeostasis
Adrenal Cortex	Cortisol, Aldosterone	Stress response, electrolyte balance
Adrenal Medulla	Adrenaline, Noradrenaline	Fight-or-flight response
Pancreas (Islets)	Insulin, Glucagon	Blood glucose regulation
Ovaries	Oestrogen, Progesterone	Female reproductive cycle
Testes	Testosterone	Male reproductive development

Key Concept: Endocrine vs Exocrine — endocrine glands lack ducts (ductless), while exocrine glands (sweat, salivary) release through ducts. ⚡ Exam tip: In MDCAT, always remember the pituitary gland is the “master gland” controlling all other endocrine glands — it’s a favourite short-answer topic.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Endocrine System — Detailed Study Guide

Hormone Mechanism — Lock and Key Model Hormones are specific to target cells because target cells have specific receptor proteins. A hormone cannot affect a cell that lacks its specific receptor — this is why thyroid hormone affects metabolic cells but not neurons directly.

Second Messenger System (cAMP) Many protein hormones (e.g., adrenaline, glucagon) cannot cross the cell membrane. They bind to receptor sites on the cell surface and activate a second messenger system:

1. Hormone binds to receptor → activates G-protein
2. G-protein activates adenylyl cyclase
3. Adenylyl cyclase converts ATP → cAMP
4. cAMP activates protein kinases → cellular response

This explains why one hormone molecule can activate many enzyme molecules — an amplification effect.

Negative Feedback Loops Most endocrine pathways operate via negative feedback. Example —Thyroid axis (HPT axis): Hypothalamus releases TRH → Pituitary releases TSH → Thyroid releases T3/T4 → High T3/T4 feedback inhibits Hypothalamus and Pituitary.

Comparison: Nervous vs Endocrine System

Feature	Nervous System	Endocrine System
Speed	Fast (milliseconds)	Slow (seconds to hours)
Duration	Short-lived	Long-lasting
Transmission	Electrical impulse	Blood-borne hormones
Coverage	Local (specific)	Wide (systemic)

⚡ Exam tip: MDCAT frequently asks about the difference between endocrine and exocrine glands — know that endocrine = ductless and uses bloodstream.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Endocrine System — Full Theory for MDCAT

Detailed Gland-by-Gland Analysis

1. Hypothalamus The hypothalamus acts as the link between the nervous system and endocrine system. It receives signals from the brain and responds by producing releasing hormones that travel to the anterior pituitary via the hypothalamo-hypophyseal portal system. Key hormones: GnRH (gonadotropin-releasing hormone), CRH (corticotropin-releasing hormone), TRH (thyrotropin-releasing hormone).

2. Pituitary Gland Anterior Pituitary — Trophic hormones:

• GH (Growth Hormone): Promotes protein synthesis and cell division. Deficiency in children → pituitary dwarfism; excess → giantism (child) or acromegaly (adult).
• TSH (Thyroid Stimulating Hormone): Stimulates thyroid hormone secretion.
• ACTH (Adrenocorticotropic Hormone): Stimulates adrenal cortex to release cortisol.
• FSH and LH: Gonadotropins controlling ovarian and testicular function.

Posterior Pituitary — Stores and releases:

• ADH (Antidiuretic Hormone/Vasopressin): Increases water reabsorption in kidneys. Deficiency → diabetes insipidus (large volumes of dilute urine).
• Oxytocin: Stimulates uterine contraction during labour and milk ejection reflex.

3. Thyroid Gland The thyroid produces T3 (triiodothyronine) and T4 (thyroxine), which regulate basal metabolic rate (BMR).

• Iodine is essential for synthesis — deficiency causes goitre (enlarged thyroid) and cretinism in children.
• T3 is the active form; T4 is converted to T3 in tissues. Calcitonin (from C cells) lowers blood calcium levels by promoting calcium deposition in bones.

Disorders:

• Hyperthyroidism (Graves’ disease): Weight loss, heat intolerance, exophthalmos, goitre.
• Hypothyroidism (Hashimoto’s): Weight gain, cold intolerance, myxoedema.

4. Parathyroid Gland Four small glands behind the thyroid. Parathyroid hormone (PTH) raises blood calcium:

• Stimulates osteoclast activity (bone resorption)
• Increases renal calcium reabsorption
• Activates vitamin D (calcitriol) in kidneys → increased intestinal calcium absorption

5. Adrenal Gland Cortex (zones from outside in):

• Zona glomerulosa: Aldosterone (mineralocorticoid) — Na⁺ retention, K⁺ excretion.
• Zona fasciculata: Cortisol (glucocorticoid) — gluconeogenesis, anti-inflammatory.
• Zona reticularis: Androgens (sex hormones).

Medulla: Chromaffin cells produce adrenaline (epinephrine) and noradrenaline — fight-or-flight response.

6. Pancreas (Islets of Langerhans)

• Alpha cells: Glucagon → raises blood glucose (glycogenolysis, gluconeogenesis).
• Beta cells: Insulin → lowers blood glucose (promotes glucose uptake, glycogenesis).
• Delta cells: Somatostatin → inhibits insulin and glucagon release.

Diabetes mellitus: Type 1 (autoimmune destruction of beta cells, insulin-dependent) vs Type 2 (insulin resistance, non-insulin-dependent).

7. Pineal Gland Melatonin — regulates circadian rhythm (sleep-wake cycle). Secretion is inhibited by light and stimulated by darkness.

Previous Year MDCAT Patterns

• 2022: MCQ on pituitary hormones — students must identify trophic vs non-trophic hormones.
• 2023: Question on negative feedback mechanism of thyroid hormones.
• 2024: Case-based question on diabetes mellitus Type 1 vs Type 2 differences.

⚡ Exam tip: In MDCAT, the hypothalamus-pituitary-thyroid axis and the role of insulin/glucagon in blood glucose are the two most frequently tested endocrine concepts.

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