Hormones: Structure, Function, Classification
What hormones are
Hormones are chemical messengers released by endocrine glands. They move through the bloodstream and act on distant target cells to regulate major body functions like growth, metabolism, reproduction, stress response, and homeostasis.
Where they are produced
Endocrine organs include the pituitary, thyroid, adrenal glands, pancreas, gonads, hypothalamus, and others. Some tissues like fat, gut, kidney, and heart also produce hormones.
General functions
- Maintain internal balance (homeostasis)
- Regulate growth and development
- Control reproduction and sexual functions
- Influence metabolism and energy use
- CoMade of aminoordinate behavior and responses to stress
Structural classification
Protein and Peptide Hormones
- acids (small peptides like ADH, or large proteins like insulin)
- Water-soluble
- Cannot cross cell membranes
- Bind cell-surface receptors
- Activate second messenger systems
Steroid Hormones
- Derived from cholesterol
- Lipid-soluble
- Easily cross cell membranes
- Bind intracellular receptors (cytoplasmic or nuclear)
- Modify gene transcription
- Includes cortisol, aldosterone, estrogen, testosterone
Amine Hormones
- Derived from tyrosine or tryptophan
- Can behave like peptide hormones (e.g., adrenaline: water-soluble, cell-surface action) or like steroid hormones (e.g., thyroid hormones: lipid-soluble, nuclear action)
Classification by where they act
Tropic hormones
- They regulate other endocrine glands
- Example: TSH stimulates thyroid, ACTH stimulates adrenal cortex
Non-tropic hormones
- Act directly on target organs
- Example: insulin on liver and muscle
Local hormones (paracrine/autocrine)
- Act near where they are produced
- Example: prostaglandins, nitric oxide
Classification by mechanism of action
Hormones acting via membrane receptors (usually protein/peptide hormones)
- Cannot enter the cell
- Bind to receptors on cell surface
- Trigger second messenger cascades (cAMP, IP3, DAG, Ca²⁺)
- Effects are fast (seconds to minutes)
Hormones acting via intracellular receptors (usually steroids or thyroid hormones)
- Enter the cell
- Bind receptors in cytoplasm or nucleus
- Affect gene transcription and protein synthesis
- Effects are slow but long-lasting
How the body controls hormone levels
- Negative Feedback (main mechanism)
Increased hormone level inhibits further release
Example: High cortisol suppresses ACTH
- Positive Feedback (rare)
Hormone increases its own release
Example: Oxytocin during childbirth
- Circadian and pulsatile rhythms
Many hormones aren’t secreted constantly but in patterns
Example: Cortisol peaks in early morning