Table of Contents
Hormones in vertebrates and humans can be grouped in several useful ways. In this chapter, the focus is on how hormones are classified, not on the detailed workings of individual glands or feedback loops (those belong in the parent chapter).
The most important classifications are:
- By chemical structure
- By solubility and transport form
- By mechanism of action at the target cell
1. Classification by Chemical Structure
1.1 Peptide and Protein Hormones
Definition and composition
- Built from chains of amino acids.
- Range from small peptides of a few amino acids to large proteins.
- Are encoded by genes, synthesized as proteins in endocrine cells.
Typical examples
- Very short peptides: oxytocin, vasopressin (ADH)
- Medium peptides: calcitonin, parathyroid hormone (PTH)
- Protein hormones: growth hormone (GH), prolactin, insulin, glucagon
- Glycoprotein hormones (proteins with attached sugar chains): TSH, LH, FSH
Key properties
- Generally water‑soluble.
- Cannot pass through the hydrophobic core of the cell membrane.
- Stored in secretory vesicles and released by exocytosis when the cell is stimulated.
1.2 Steroid Hormones
Definition and composition
- Derived from the lipid cholesterol.
- Share a characteristic four-ring steroid skeleton.
- Modified by adding or removing specific groups (e.g., hydroxyl, keto, methyl).
Main groups in vertebrates
- Glucocorticoids: e.g., cortisol
- Mineralocorticoids: e.g., aldosterone
- Sex steroids:
- Androgens: e.g., testosterone
- Estrogens: e.g., estradiol
- Gestagens (progestagens): e.g., progesterone
Key properties
- Lipid‑soluble (hydrophobic).
- Can diffuse through cell membranes.
- Not stored in vesicles in large amounts; mainly synthesized and released on demand.
1.3 Amino Acid–Derived (Amine) Hormones
These arise by chemical modification of single amino acids, especially tyrosine and tryptophan.
Tyrosine derivatives
- Catecholamines (water‑soluble):
- Adrenaline (epinephrine)
- Noradrenaline (norepinephrine)
- Dopamine
- Thyroid hormones (lipid‑like, but with special transport):
- Thyroxine (T4)
- Triiodothyronine (T3)
Tryptophan and other derivatives
- Melatonin (from tryptophan)
- Histamine (from histidine; locally acting in many tissues)
Key properties
- Catecholamines behave similarly to peptide hormones: water‑soluble, act via membrane receptors.
- Thyroid hormones behave more like steroid hormones: cross membranes and act primarily on nuclear receptors, but need special transport proteins in blood.
1.4 Eicosanoids and Other Lipid Derivatives
Definition
- Derived from polyunsaturated fatty acids, especially arachidonic acid.
- Often act as local hormones rather than systemic long‑range hormones.
Important families
- Prostaglandins
- Thromboxanes
- Leukotrienes
Key properties
- Usually very short‑lived.
- Act locally (paracrine, autocrine).
- Involved in processes like inflammation, blood clotting, and smooth muscle contraction.
2. Classification by Solubility and Transport Form
A practical classification divides hormones into hydrophilic (water‑soluble) and lipophilic (fat‑soluble), because solubility determines how they travel in the blood and how they reach their receptors.
2.1 Hydrophilic Hormones
Include
- Most peptide and protein hormones
- Catecholamines
- Some small water‑soluble molecules
Transport and storage
- Circulate freely dissolved in blood plasma.
- Often stored in secretory vesicles inside endocrine cells.
Target cell entry
- Cannot pass the lipid bilayer easily.
- Bind to membrane receptors on the cell surface.
Consequence
- Tend to trigger rapid responses (seconds to minutes) via signal cascades inside the cell.
2.2 Lipophilic Hormones
Include
- Steroid hormones
- Thyroid hormones
- Many eicosanoids
Transport and storage
- Poorly soluble in water.
- In blood, are usually bound to transport proteins (e.g., albumin, specific binding globulins).
- Within endocrine cells, often not stored in granules; synthesized and released as needed.
Target cell entry
- Can usually diffuse through cell membranes.
- Free (unbound) fraction is biologically active and enters cells.
Consequence
- Often change gene expression, with effects that develop more slowly (hours to days) but can last longer.
3. Classification by Mechanism of Action
The type of receptor and signal transmission in the target cell provides another important grouping.
3.1 Hormones Acting via Membrane Receptors
These hormones do not enter the cell; they bind to proteins on the cell surface.
Typical members
- Peptide and protein hormones
- Catecholamines
- Some locally acting amines and peptides
Receptor types (examples)
- G‑protein–coupled receptors (GPCRs)
- Receptor tyrosine kinases (RTKs)
- Receptor guanylyl cyclases
- Ion channel–linked receptors
Second messengers
Binding of the hormone often triggers intracellular second messengers, such as:
- cAMP
- cGMP
- Inositol trisphosphate (IP₃)
- Diacylglycerol (DAG)
- Ca²⁺ ions
Typical effects
- Rapid changes in enzyme activity
- Opening or closing of ion channels
- Modulation of existing signaling pathways
These actions commonly regulate short‑term adjustments, such as heart rate, smooth muscle tone, secretion, or glucose uptake.
3.2 Hormones Acting via Intracellular Receptors
These hormones are lipophilic and can cross membranes. Their receptors are inside the cell:
- In the cytoplasm, then move into the nucleus, or
- Directly in the cell nucleus.
Typical members
- Steroid hormones (cortisol, aldosterone, sex hormones)
- Thyroid hormones
- Some vitamin‑derived regulators (e.g., active vitamin D form)
Mode of action
- Hormone + receptor form a complex.
- This complex binds to specific DNA sequences (hormone response elements).
- This changes transcription of certain genes (up‑ or downregulation).
- Altered gene expression leads to changes in protein composition in the cell.
Typical effects
- Slower onset but long‑lasting changes in:
- Cell growth and division
- Differentiation
- Metabolic enzyme patterns
- Developmental processes and long‑term homeostasis
4. Local vs. Systemic Action
Although the focus is on endocrine (blood‑borne) hormones, classification can also consider the range of action.
4.1 Endocrine Hormones
- Secreted into the bloodstream.
- Act on distant target organs.
- Classic hormones of vertebrate glands (pituitary, thyroid, adrenal cortex, etc.) belong here.
4.2 Paracrine and Autocrine Hormones
- Paracrine: act on neighboring cells in the same tissue (e.g., many growth factors, eicosanoids).
- Autocrine: act on the same cell that secretes them (common in immune cells and some tumor cells).
Many substances can play multiple roles (hormone, paracrine factor, neurotransmitter), so they are sometimes called signaling molecules or messenger substances rather than strictly “hormones” in the narrow sense.
5. Summary of the Main Classification Axes
For orientation, the most widely used classification dimensions are:
- Chemical nature
- Peptide/protein hormones
- Steroid hormones
- Amino acid–derived (amines)
- Eicosanoids and other lipid derivatives
- Solubility and transport
- Hydrophilic: free in plasma, membrane receptors
- Lipophilic: protein‑bound in plasma, intracellular receptors
- Mechanism of action
- Via cell surface receptors and second messengers (fast, short‑term)
- Via intracellular receptors affecting gene expression (slower, long‑term)
These classifications help predict how a hormone is stored, transported, how quickly it acts, through which receptors it signals, and what kind of effects it produces in vertebrates and humans.