Hormones
Chemical messengers released into the blood that act on distant cells carrying the right receptor, coordinating growth, metabolism, stress and reproduction over minutes to years.
Hormones come in families. Peptides and proteins (insulin, growth hormone, thyroid-stimulating hormone) act on surface receptors and work within minutes. Steroids (cortisol, aldosterone, testosterone, oestrogen) are made from cholesterol, pass into cells and switch genes on or off over hours. Amines include adrenaline, fast and short-lived, and thyroid hormone, slow and long-lasting.
The same hormone can do different things in different tissues: adrenaline speeds the heart, dilates airways and releases glucose all at once, which is exactly what a fight-or-flight response needs. Concentrations are tiny, measured in picomoles, so small changes matter and assays must be precise.
Because hormones reach everywhere, endocrine disease is multi-system: an overactive thyroid causes weight loss, tremor, palpitations, heat intolerance and anxiety together.
Key facts
| Fact | Value |
|---|---|
| Hormone classes | peptide, steroid, amine |
| Typical blood concentration | pmol–nmol/L |
| Thyroid hormone half-life (T4) | ≈7 days |