Hormone – Overview
A hormone is a chemical messenger secreted by an endocrine gland directly into the bloodstream, where it travels to distant target organs and regulates their activity. Hormones operate on much longer timescales than neurotransmitters — from minutes to days — and their effects can be systemic, reaching virtually every cell in the body.
Major hormone-producing glands of the human endocrine system (Wikimedia Commons, CC BY-SA 4.0)
Hormones don’t determine, command, cause, or invent behaviors. Instead they make us more sensitive to social triggers of emotionally laden behaviors and exaggerate our preexisting tendencies in those domains. (→ 052 🫧Hormone und Neurotransmitter)
Classification by Chemical Structure
1. Steroid Hormones (lipid-soluble)
Derived from Cholesterol — can cross cell membranes and bind to intracellular/nuclear receptors, directly altering gene expression. Slowest but longest-lasting effects.
- Androgens — Testosterone (male sex characteristics, muscle, dominance)
- Estrogens — Estradiol (female reproductive system, bone density)
- Progestins — Progesterone (menstrual cycle, pregnancy)
- Glucocorticoids — Cortisol (stress response, metabolism, immune suppression)
- Mineralocorticoids — Aldosterone (water/salt balance, blood pressure)
→ Full detail: steroid hormone
2. Peptide Hormones (water-soluble)
Chains of amino acids — cannot cross membranes, bind to surface receptors (GPCRs or receptor tyrosine kinases), triggering second messenger cascades. Faster onset than steroids.
- Growth Hormone (GH) — growth, metabolism
- Prolactin — milk production, social bonding
- Insulin / Glucagon — blood glucose regulation
- ADH (Antidiuretic Hormone = Vasopressin) — water retention
- Oxytocin — uterine contraction, bonding (dual: also neuropeptide)
3. Amine Hormones
Derived from single amino acids (tyrosine or tryptophan):
- Epinephrine (Adrenaline) & Norepinephrine — adrenal medulla, fight-or-flight
- Thyroid hormones (T3/T4) — metabolism rate, development
- Melatonin — circadian rhythm, sleep
Signaling Modes
| Mode | Description | Example |
|---|---|---|
| Endocrine | Released into blood → distant target | Cortisol from adrenal → liver |
| Paracrine | Acts on nearby cells | Prostaglandins at injury site |
| Autocrine | Acts on the releasing cell itself | Growth factors in cancer |
| Neurocrine | Neuron releases hormone into blood | Oxytocin from hypothalamus → pituitary → blood |
The HPA Axis — A Classic Hormone Cascade
The Hypothalamic-Pituitary-Adrenal (HPA) axis shows how hormones chain together:
Stressor
↓
Hypothalamus → releases CRH ([[CRH]], a neuropeptide)
↓
Anterior Pituitary → releases ACTH (peptide hormone)
↓
Adrenal Cortex → releases Cortisol (steroid hormone)
↓
Target tissues (liver, immune system, brain) → stress response
↓
Negative feedback: cortisol inhibits hypothalamus + pituitary
This cascade spans seconds to hours from initial CRH release to peak cortisol. The same logic applies to HPG (reproductive) and HPT (thyroid) axes.
Regulation: Feedback Loops
Most hormones are controlled by negative feedback:
- High cortisol → suppresses CRH and ACTH → cortisol drops
- High T3/T4 → suppresses TSH → thyroid slows
Positive feedback is rare but powerful:
- Estrogen peak before ovulation → triggers LH surge → ovulation
Key Examples from Vault
| Hormone | Type | Primary Function | Note |
|---|---|---|---|
| Testosterone | Steroid | Sex characteristics, dominance, libido | Testosterone |
| Progesterone | Steroid | Pregnancy maintenance, cycle regulation | Progesterone |
| Epinephrine | Amine | Fight-or-flight, arousal | Epinephrine |
| Growth Hormone | Peptide | Growth, metabolism, tissue repair | Growth Hormone |
| Prolactin | Peptide | Lactation, social bonding | Prolactin |
| Glucagon | Peptide | Raises blood glucose (anti-insulin) | Glucagon |
| Vasopressin/ADH | Peptide | Water retention, social behavior | Vasopressin |
Dual-Function Molecules
Several molecules act as both neuropeptides and hormones depending on release site:
- Oxytocin — neuropeptide in hypothalamic circuits (social behavior) + hormone in blood (uterine contraction, milk letdown)
- Vasopressin — neuropeptide in brain (social memory, pair bonding) + hormone in blood (ADH, water retention)
- Epinephrine — neurotransmitter in CNS + hormone from adrenal medulla
→ Full comparison: Neurotransmitter vs Neuropeptide vs Hormone
Hormones vs. Neurotransmitters — Key Takeaway
| Feature | Hormone | Neurotransmitter |
|---|---|---|
| Travel distance | Systemic (bloodstream) | Local (synaptic cleft) |
| Speed | Minutes–days | Milliseconds |
| Source | Endocrine gland | Neuron |
| Specificity | Target cells with right receptor | Postsynaptic neuron |
see also
Tags: HormoneNeurotransmitter biology medicine science
Superlink: 052 🫧Hormone und Neurotransmitter 051 ☣Neurobiology 050 🧠Neuroscience
Linked notes: steroid hormone · Testosterone · Progesterone · Epinephrine · Growth Hormone · Prolactin · Glucagon · Vasopressin · Oxytocin · CRH · Cholesterol · Neurotransmitter · Neuropeptide · Neurotransmitter vs Neuropeptide vs Hormone
Quellen / Sources
Endocrine system – Wikipedia
Endocrine system diagram – Wikimedia Commons
Hormone – Wikipedia
Created: 14/06/26