Hormones: Types, Mechanisms, and Effects

Hormones: Types, Mechanisms, and Effects

Types of Hormones

Hormones are chemical messengers that regulate physiological processes in the body. They can be classified based on their solubility and chemical structure.

• Lipid-soluble hormones: These hormones require transporter proteins to circulate in the bloodstream due to their hydrophobic nature.

  • Steroid hormones: Derived from cholesterol; examples include cortisol, aldosterone, and sex hormones.

  • Thyroid hormones: Produced by the thyroid gland; primarily thyroxine (T4) and triiodothyronine (T3).

  • Nitric oxide (NO): A gaseous signaling molecule with hormone-like effects.

• Water-soluble hormones: These hormones circulate freely in the plasma and do not require carrier proteins.

  • Amine hormones: Derived from amino acids; examples include epinephrine and norepinephrine.

  • Peptide/protein/glycoprotein hormones: Composed of chains of amino acids; examples include insulin, growth hormone, and parathyroid hormone.

Mechanisms of Hormone Action

The physiological response to a hormone depends on both the hormone itself and the characteristics of the target cell.

• Lipid-soluble hormones: Bind to intracellular receptors located within the target cell, often in the cytoplasm or nucleus. This typically leads to changes in gene expression.

• Water-soluble hormones: Bind to receptors on the plasma membrane of target cells. This activates a second messenger system (such as cyclic AMP), which amplifies the signal inside the cell.

Factors Affecting Target Cell Responsiveness

• Hormone concentration: Higher concentrations generally produce greater effects.

• Abundance of target cell receptors: More receptors increase sensitivity to the hormone.

• Influence of other hormones: The presence of other hormones can modify the response.

Interactions Between Hormones

Hormones can interact in several ways to regulate physiological processes:

• Permissive effect: One hormone enhances the effect of another. Example: Cortisol increases the effect of glucagon on blood glucose levels.

• Synergistic effect: Two hormones work together to produce a greater effect than either alone. Example: Antidiuretic hormone (ADH) and aldosterone both increase water reabsorption in the kidneys.

• Antagonistic effect: One hormone opposes the action of another. Example: Parathyroid hormone (PTH) increases blood calcium levels, while calcitonin decreases them.

Summary Table: Hormone Types and Examples

Hormone Type	Solubility	Examples	Transport in Blood
Steroid	Lipid-soluble	Cortisol, Aldosterone, Sex hormones	Carrier proteins
Thyroid	Lipid-soluble	Thyroxine (T4), Triiodothyronine (T3)	Carrier proteins
Nitric oxide (NO)	Lipid-soluble	Nitric oxide	Carrier proteins
Amine	Water-soluble	Epinephrine, Norepinephrine	Free form
Peptide/Protein/Glycoprotein	Water-soluble	Insulin, Growth hormone, Parathyroid hormone	Free form

Key Terms

• Second messenger system: Intracellular signaling molecules (e.g., cAMP) that mediate the effects of water-soluble hormones.

• Receptor: A protein on or within a cell that binds a specific hormone and initiates a response.

• Permissive, synergistic, antagonistic effects: Types of hormone interactions that modulate physiological outcomes.

Additional info: Expanded definitions and examples were added for clarity and completeness.