Endocrine Glands and Hormones — Revision Notes

⚡ 30-Second Revision

Endocrine Glands: — Ductless, secrete hormones directly into blood.
Hormones: — Chemical messengers, act on target cells with specific receptors.
Types: — Peptides (insulin, glucagon, GH) \rightarrow Membrane receptors, second messengers (cAMP). Steroids (cortisol, estrogen, testosterone) \rightarrow Intracellular receptors, gene expression. Thyroid hormones (T3, T4) \rightarrow Lipid-soluble, intracellular receptors.
Hypothalamus: — Releasing/Inhibiting hormones (GnRH, TRH), ADH, Oxytocin (stored in posterior pituitary).
Pituitary: — Anterior (GH, PRL, TSH, ACTH, FSH, LH), Posterior (ADH, Oxytocin).
Thyroid: — T3, T4 (metabolism), Calcitonin (\downarrow Ca\textsuperscript{2+} in blood).
Parathyroid: — PTH (\uparrow Ca\textsuperscript{2+} in blood).
Adrenal Cortex: — Cortisol (glucocorticoid), Aldosterone (mineralocorticoid), Androgens.
Adrenal Medulla: — Adrenaline, Noradrenaline (catecholamines).
Pancreas (Islets): — Insulin (\downarrow blood glucose), Glucagon (\uparrow blood glucose).
Gonads: — Testes (Testosterone), Ovaries (Estrogen, Progesterone).
Pineal: — Melatonin (sleep-wake cycle).
Thymus: — Thymosins (immunity).
Regulation: — Primarily negative feedback.

2-Minute Revision

The endocrine system is your body's chemical communication network, utilizing hormones secreted by ductless glands directly into the bloodstream. These hormones are specific messengers, acting only on target cells equipped with matching receptors.

Hormones are broadly categorized into peptides (like insulin, acting via membrane receptors and second messengers) and steroids (like estrogen, acting via intracellular receptors to alter gene expression).

Thyroid hormones, though amino acid derivatives, also act intracellularly. Key glands include the hypothalamus and pituitary, which regulate many other glands. The thyroid controls metabolism (T3, T4), while parathyroid hormone (PTH) and calcitonin manage calcium levels.

The adrenal glands handle stress (cortisol, adrenaline) and electrolyte balance (aldosterone). The pancreas regulates blood sugar with insulin and glucagon, and the gonads produce sex hormones. The pineal gland secretes melatonin for sleep, and the thymus aids immunity.

This entire system is tightly controlled by feedback loops, predominantly negative feedback, to maintain homeostasis. Understanding each gland's hormones, their functions, and the consequences of their imbalance is crucial for NEET.

5-Minute Revision

The endocrine system is a crucial regulatory system, working alongside the nervous system to maintain homeostasis through chemical coordination. It consists of ductless glands that release hormones directly into the bloodstream.

Hormones are non-nutrient chemical messengers that act on specific target cells possessing complementary receptors. Their chemical nature dictates their mechanism of action: peptide hormones (e.g., insulin, growth hormone) are water-soluble, binding to membrane receptors and initiating intracellular signaling cascades via second messengers (like cAMP).

Steroid hormones (e.g., cortisol, estrogen) and thyroid hormones (T3, T4) are lipid-soluble, diffusing across the cell membrane to bind with intracellular receptors, forming a hormone-receptor complex that directly influences gene transcription in the nucleus.

Major endocrine glands include:

Hypothalamus: — Produces releasing and inhibiting hormones that control the anterior pituitary, and synthesizes ADH and Oxytocin for posterior pituitary release.
Pituitary Gland: — The 'master gland,' with an anterior lobe (GH, PRL, TSH, ACTH, FSH, LH) and a posterior lobe (releases ADH, Oxytocin).
Pineal Gland: — Secretes Melatonin, regulating circadian rhythms.
Thyroid Gland: — Produces T3 and T4 (metabolism, growth) and Calcitonin (lowers blood Ca\textsuperscript{2+}).
Parathyroid Glands: — Secrete Parathyroid Hormone (PTH), which raises blood Ca\textsuperscript{2+}.
Thymus: — Secretes Thymosins, crucial for T-lymphocyte maturation.
Adrenal Glands: — Cortex secretes glucocorticoids (cortisol), mineralocorticoids (aldosterone), and adrenal androgens. Medulla secretes catecholamines (adrenaline, noradrenaline).
Pancreas (Islets of Langerhans): — Alpha cells secrete Glucagon (raises blood glucose), Beta cells secrete Insulin (lowers blood glucose).
Gonads: — Testes produce Testosterone; Ovaries produce Estrogen and Progesterone.

Hormone secretion is primarily regulated by negative feedback loops, where the end product inhibits its own production, ensuring stable levels. Positive feedback, though rarer, amplifies a response (e.g., oxytocin in childbirth). Understanding the specific functions of each hormone and the symptoms of their hypo- or hyper-secretion (e.g., diabetes, thyroid disorders, growth disorders) is critical for NEET, as clinical scenarios are frequently tested.

Prelims Revision Notes

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Endocrine vs. Exocrine: — Endocrine glands are ductless, secrete hormones into blood. Exocrine glands have ducts, secrete products onto surfaces/cavities.
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Hormone Nature & Action:

* Peptide/Protein Hormones (Insulin, Glucagon, GH, ADH, Oxytocin, Pituitary hormones): Water-soluble. Bind to membrane receptors. Use second messengers (cAMP, IP3, Ca\textsuperscript{2+}).

Rapid, short-lived effects. * Steroid Hormones (Cortisol, Aldosterone, Estrogen, Testosterone, Progesterone): Lipid-soluble. Bind to intracellular receptors (cytoplasm/nucleus). Hormone-receptor complex binds to DNA (HREs), alters gene expression.

Slower, long-lasting effects. * Thyroid Hormones (T3, T4): Amino acid derivatives, but lipid-soluble. Act like steroids via intracellular receptors. * Catecholamines (Adrenaline, Noradrenaline): Amino acid derivatives.

Water-soluble. Bind to membrane receptors.

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Hypothalamus: — Neurosecretory cells produce releasing (e.g., GnRH, TRH) and inhibiting (e.g., Somatostatin) hormones for anterior pituitary. Synthesizes ADH and Oxytocin, stored in posterior pituitary.
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Pituitary Gland:

* Anterior Pituitary: GH (growth), PRL (milk production), TSH (thyroid stim), ACTH (adrenal cortex stim), FSH (gonad stim), LH (gonad stim). * Posterior Pituitary: Releases ADH (vasopressin - water reabsorption), Oxytocin (uterine contractions, milk ejection).

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Pineal Gland: — Melatonin (sleep-wake cycles, body temperature).
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Thyroid Gland: — T3 (triiodothyronine), T4 (thyroxine) - regulate BMR, growth, development. Calcitonin - lowers blood Ca\textsuperscript{2+} (antagonistic to PTH).

* Disorders: Hypothyroidism (Cretinism in children, Myxedema in adults), Hyperthyroidism (Grave's disease - exophthalmos).

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Parathyroid Glands: — Parathyroid Hormone (PTH) - raises blood Ca\textsuperscript{2+} (bone resorption, renal reabsorption, vit D activation).
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Thymus: — Thymosins - T-lymphocyte maturation, immunity.
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Adrenal Glands:

* Cortex: Glucocorticoids (Cortisol - stress, glucose metabolism, anti-inflammatory), Mineralocorticoids (Aldosterone - Na\textsuperscript{+}, K\textsuperscript{+} balance, BP), Adrenal Androgens. * Medulla: Adrenaline (epinephrine), Noradrenaline (norepinephrine) - 'fight or flight' response. * Disorders: Cushing's (excess cortisol), Addison's (deficient cortisol/aldosterone).

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Pancreas (Islets of Langerhans):

* Alpha cells: Glucagon - raises blood glucose (glycogenolysis, gluconeogenesis). * Beta cells: Insulin - lowers blood glucose (glucose uptake, glycogenesis). * Disorders: Diabetes Mellitus (Type 1 - insulin deficiency, Type 2 - insulin resistance).

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Gonads:

* Testes: Testosterone (spermatogenesis, male secondary sex chars). * Ovaries: Estrogen (female secondary sex chars, follicular development), Progesterone (pregnancy, menstrual cycle).

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Hormones from other organs: — ANF (heart - \downarrow BP), Erythropoietin (kidney - RBC formation), Gastrin, Secretin, CCK, GIP (GI tract - digestion).
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Regulation: — Mostly negative feedback (e.g., T3/T4 inhibiting TSH/TRH). Positive feedback (e.g., Oxytocin in childbirth) is rare.

Vyyuha Quick Recall

To remember the hormones of the Anterior Pituitary: Flat Pig Gets To All Lovely Men.

FSH (Follicle Stimulating Hormone)
PRL (Prolactin)
GH (Growth Hormone)
TSH (Thyroid Stimulating Hormone)
ACTH (Adrenocorticotropic Hormone)
LH (Luteinizing Hormone)
(M - Melatonin is from Pineal, not Anterior Pituitary, but sometimes included in broader pituitary mnemonics for completeness, though it's a common point of confusion. For strict anterior pituitary, just remember the first six.)