Different Types of Hormones

Hormones are any chemical messengers which have a specific effect upon specific types of cells. (eg. Growth regulators are one type of hormone hormones).  Hormones can be broadly classified based on their final destination:

  1. Autocrine
    Hormones that act on the cell that made them. 
  2.   Paracrine
        Hormones that act on cells adjacent to the cell that made them
  3. Endocrine
    Hormones that act on cells remote from the cell that made them (they are transported via the bloodstream)

Hormones are a functionally related group of biomolecules, however they are structurally diverse.  Some are lipophilic, such as the steroid hormones, and many of these are small enough that they can enter a cell straight through their plasma membranes (the phospholipid bilayer surrounding the cell).  Many other hormones are large and hydrophilic and either transmit their signal via a receptor protein in the plasma membrane or by entering through special gated channels in the membrane.  The general function of all hormones is to affect some function of the target cell.  The specifics of how cells respond to a hormone are beyond the scope of this introductory course.  Hormones regulate the growth, metabolism, sexual development and reproduction of all multicellular living organisms, along with several other processes, including immune system function, bone density, the fight or flight response, brain function, the storage and mobilisation of fat from adipose tissue and so on.

Biochemically, hormones are classified as either:

  • Amine hormones (derived from amino acids, these are nitrogenous hormones)
  • Lipid hormones (lipid based molecules including steroid hormones
  • Peptide hormones (proteins that have hormone function, they may be purely amino acid based, or they may be conjugates of protein and another type of organic molecule such as carbohydrate)

While almost all cells produce hormones, there are also specific tissues in the body, known as glands, which produce a variety of endocrine hormones.  These glands make up the endocrine system.  The adipose (fat) tissue is often included as a part of the endocrine system because it too produces a range of endocrine hormones.  Glands may secrete hormone molecules directly into the blood stream (true endocrine hormones) or they may release them via a duct into the surrounding extracellular fluid or blood stream (exocrine hormones).