# Chemical Nature of Hormones > Chemical nature of hormones for NEET Biology: peptide, steroid, iodothyronine and catecholamine classes, receptor location and second messengers. - Canonical URL: https://prepelephant.com/topics/neet-ug/biology/hormones-chemical-nature-neet - Exam / course: NEET-UG · Subject: Biology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Chemical Nature of Hormones", PrepElephant, https://prepelephant.com/topics/neet-ug/biology/hormones-chemical-nature-neet ## Direct answer Chemistry predicts how a hormone behaves: NCERT classifies human hormones into four chemical classes, and each class carries its own mechanism. Peptide, polypeptide and protein hormones — insulin, glucagon, the hypothalamic and pituitary hormones — are water-soluble and bind receptors on the cell surface, generating second messengers inside. Steroid hormones — cortisol, aldosterone, testosterone, estradiol, progesterone — derive from cholesterol, slip through cell membranes and act on intracellular receptors, largely regulating gene expression. The iodothyronines, thyroxine (T4) and triiodothyronine (T3), are iodinated amino-acid derivatives that also act through intracellular receptors, and the catecholamines adrenaline and noradrenaline, derived from the amino acid tyrosine, act at surface receptors like the peptide class. ## What you must remember - **Four classes, verbatim:** (i) peptide, polypeptide, protein hormones; (ii) steroids; (iii) iodothyronines; (iv) amino-acid-derived catecholamines — NCERT's own grouping, so learn it as printed. - **Class membership map:** insulin, glucagon, pituitary and hypothalamic hormones = peptides; cortisol, aldosterone, testosterone, estradiol, progesterone = steroids from cholesterol; T3 and T4 = iodothyronines; adrenaline and noradrenaline = catecholamines from tyrosine. - **Mechanism divide:** peptide hormones act through membrane receptors and second messengers such as cyclic AMP; steroid hormones bind intracellular (mostly nuclear) receptors and modulate transcription — the pair of statements NEET builds its assertion-reason items on. - **Adrenaline's double life:** a catecholamine from the adrenal medulla and the emergency "fight or flight" hormone, increasing heart rate and mobilising glucose — never a steroid, whatever the option implies. - **Pituitary split:** anterior lobe secretes FSH, LH, ACTH, TSH, prolactin and growth hormone; posterior lobe releases only oxytocin and vasopressin (ADH), which are peptide hormones made in the hypothalamus. - **Non-endocrine sources:** the heart's atria release ANF, the kidney makes erythropoietin, and the GI tract secretes hormones — endocrine signalling outside classic glands. - **Pineal timing:** melatonin, from the pineal gland, regulates the 24-hour circadian rhythm and influences pigmentation in lower vertebrates — the biological-clock answer. ## Reason from structure to mechanism with two hormones Take insulin, a small protein. It cannot cross the lipid bilayer, so it docks on a surface receptor; the engaged receptor relays the signal through second messengers that drive GLUT transporters onto the membrane, and glucose entry rises within minutes. Now take cortisol, a cholesterol-derived steroid: lipid-soluble, it diffuses straight through the membrane and finds its receptor in the cytoplasm or nucleus, and the complex switches specific genes on or off — new proteins appear over hours, not minutes. Surface-receptor speed against nuclear-receptor stamina is why stress responses are layered, and why the same mechanism split decides drug delivery: insulin must be injected because digestion would destroy it, a clinical footnote Indian vivas commonly probe. Thyroxine completes the triangle: synthesised from tyrosine with iodine atoms attached — the reason dietary iodine deficiency devastates the gland, and the reason India iodises salt — it behaves like the steroid class at the receptor level despite its amino-acid ancestry. Adrenaline is the counter-example: also a tyrosine derivative, yet water-soluble and surface-acting, so ancestry alone does not settle the mechanism; the four-way class table does. ## Where students lose marks The adrenaline-steroid confusion is the single most planted error: it is a catecholamine, and questions expecting "steroid hormone of the adrenal medulla" lose the mark to anyone who checks. Second, mechanism wording: steroids regulate gene expression through intracellular receptors; peptides generate second messengers through surface receptors — swap either half and the statement is false. Third, thyroxine's classification: iodothyronine, a distinct NCERT class, not simply "amino acid derivative" when the four-way key is offered. And remember the posterior pituitary secretes peptides, not steroids, since oxytocin and ADH are hypothalamic peptide hormones merely stored below. ## Frequently asked questions ### Into which chemical classes does NCERT group hormones? Peptide/protein hormones, steroid hormones, iodothyronines, and amino-acid-derived catecholamines. ### How do peptide and steroid hormones differ in action? Peptide hormones bind cell-surface receptors and act through second messengers, while steroids cross membranes and regulate gene expression via intracellular receptors. ### What is the chemical nature of adrenaline and thyroxine? Adrenaline is a catecholamine derived from tyrosine; thyroxine is an iodinated derivative of tyrosine placed in the separate iodothyronine class. ### From what precursor are steroid hormones synthesised? Cholesterol, in the adrenal cortex and the gonads. ### Which hormones does the posterior pituitary release, and of what chemical type? Oxytocin and vasopressin (ADH) — peptide hormones synthesised in hypothalamic neurons and only stored and released by the posterior pituitary.