Hormones are the body’s long-distance chemical messengers. Even though well over 50 distinct hormones circulate in humans, nearly every one can be assigned to one of three core chemical classes—each with its own blueprint for synthesis, its own style of travel through the bloodstream, and its own way of “talking” to target cells.
| Hormone Class | Molecular Backbone | Solubility & Transport | Typical Receptor Site | Representative Examples | Plasma Half-Life* |
|---|---|---|---|---|---|
| Lipid-derived (steroid ± eicosanoid) | Cholesterol (27-C ring) or arachidonic acid | Hydrophobic → protein carriers (albumin, SHBG, CBG) | Intracellular (cytosolic → nuclear transcription factors) | Cortisol, aldosterone, estradiol, testosterone, progesterone; calcitriol; prostaglandins | 30 – 120 min (cortisol ≈ 60-90 min) |
| Amino-acid-derived (biogenic amines) | Single modified Tyr or Trp | Mostly water-soluble (thyroid hormones are lipid-soluble outliers) | Membrane GPCRs (Epi/NE), nuclear receptors (T₃/T₄) | Epinephrine, norepinephrine, dopamine, thyroxine (T₄), triiodothyronine (T₃), melatonin | 1-5 min (Epi), days (T₄) |
| Peptide / Protein / Glycoprotein | 3 to >200 amino acids ± carbohydrate side chains | Hydrophilic → circulate free; stored in granules until exocytosis | Plasma-membrane receptors (GPCRs, RTKs, JAK-STAT) | Insulin, glucagon, parathyroid hormone, growth hormone, oxytocin, FSH/LH/TSH | Seconds to <30 min (insulin ≈ 5-10 min) |
*Half-life values are ballpark averages in healthy adults.
1. Lipid-Derived Hormones
Building blocks & biosynthesis
- Steroids originate from cholesterol via mitochondrial & smooth-ER enzymes (CYP11A1, 3β-HSD, etc.).
- Eicosanoids (prostaglandins, leukotrienes) sprout from membrane phospholipids through phospholipase A₂ and COX/LOX pathways.
Key traits
- Passively diffuse through plasma membranes; no vesicular storage.
- Ride in blood bound to carrier proteins, which prolongs half-life and creates a hormone reservoir.
- Receptors reside inside the cell. Ligand–receptor complexes bind DNA at hormone-response elements, modulating transcription—hence effects unfold in hours to days.
2. Amino-Acid-Derived Hormones
Two molecular templates
- Tyrosine-derived catecholamines (Epi, NE, dopamine) and thyroid hormones.
- Tryptophan-derived melatonin.
Dual personalities
- Catecholamines behave like peptides: water-soluble, rapid exocytosis, GPCR signaling (Gs, Gq, Gi subtypes).
- Thyroid hormones act like steroids: lipid-soluble, bound to thyroxine-binding globulin, intracellular receptors that increase basal metabolic rate by driving mitochondrial and Na⁺/K⁺-ATPase genes.
3. Peptide & Protein Hormones
Assembly line
- Pre-prohormone in rough ER → cleavage to prohormone.
- Golgi packaging into secretory granules → final enzymatic trimming.
- Calcium-triggered exocytosis in response to physiologic stimuli (glucose for insulin, osmolarity for ADH).
Signal transduction repertoire
- GPCR → cAMP (e.g., glucagon)
- GPCR → IP₃/DAG/Ca²⁺ (e.g., oxytocin)
- Receptor tyrosine kinase (RTK) autophosphorylation (e.g., insulin)
- JAK-STAT cytokine receptors (e.g., GH, prolactin)
Because their messages are amplified by second-messenger cascades, peptide hormones often produce second-to-minute-scale physiologic changes.
4. Putting It All Together—Why Classification Matters
| Clinical Angle | Practical Implication |
|---|---|
| Route of administration | Peptides are GI-labile → injectable or nasal (insulin, desmopressin); steroids survive PO (prednisone, estradiol). |
| Onset & duration | Catecholamines = lightning fast; steroids = slower onset but longer action → tailor emergency vs. chronic therapy. |
| Diagnostic timing | Measure cortisol ≥60 min post-ACTH; sample catecholamines rapidly on ice; draw TSH/thyroid hormones in the morning for consistency. |
| Drug interactions | Albumin-binding drugs can displace steroids; MAO inhibitors boost catecholamine levels; somatostatin analogs blunt GH, insulin, glucagon. |
5. Beyond the Big Three: Honorable Mentions
- Gasotransmitters: Nitric oxide (NO) and carbon monoxide (CO) act locally, defying classical hormone rules.
- Peptide-steroid hybrids: Vitamin D₃ (calcitriol) derives from cholesterol yet acts via an intracellular receptor like a true steroid—and regulates calcium alongside PTH and calcitonin.
Key Takeaways
- Three foundational classes—lipid, amino-acid, peptide—cover virtually all systemic hormones.
- Solubility and size dictate everything: storage, transport, receptor location, signal speed, and clinical handling.
- Understanding chemistry → better medicine: from designing oral contraceptives that bypass first-pass metabolism to timing blood draws for an accurate endocrine work-up.
Armed with these chemical categories, clinicians and researchers can predict how a hormone behaves in the body, select the right diagnostic tests, and craft therapies that restore balance when endocrine harmony falters.