Hormones can be classified by where they function i.e. the autocrine function and paracrine function and endocrine function.

More importantly, hormones can be classified by structure. The structure of a hormone really determines how it works.

Proteins & Polypeptides

Proteins and polypeptides are made up of amino acids. These amino acids are linked together with peptide bonds. Many peptide bonds come together to form a polypeptide or a protein.

These proteins and polypeptides form most of our body’s hormones. These hormones can range from small to large. To give you an example of what I mean by that, imagine three or so amino acids linked together forming a hormone. That would be a small polypeptide.

Just as a frame of reference, a cell in your body, one cell, has on the order of a trillion atoms just inside that one cell. There are a hundred trillion cells in your body, so we’re talking about very, very, very small things.

They can range from these small collections of amino acids all the way The break point becomes right around a hundred. That’s where we shift from calling them polypeptides to proteins.

They can get quite large. The polypeptide hormones and proteins are formed in the rough endoplasmic reticulum of the cell.

They go from the rough endoplasmic reticulum:

  1. To the “golgi apparatus”
  2. Then they’re repackaged into vesicles that can eventually be excreted from the cell
  3. Because proteins and polypeptides are made of amino acids, they’re typically charge, which makes them water soluble, but it also gives them a really hard time crossing cell membranes, so typically, their receptors are located in or on a cell surface.
  4. Because their receptors are located in or on a cell surface, so the polypeptide hormones and proteins are not able to travel into the cell, instead, they initiate a cascade effect of secondary messengers inside the cell. The moment the proteins, as well as the polypeptide hormones attach themselves to the surface of the cell, they trigger a reaction within the cell. This is usually referred to as the “secondary messenger system”.

Amino acids are used as chemical messengers to signal effects in the body, and these are called proteins and polypeptides.


When we hear steroids, the first thought that comes to my mind are a bunch of athletes getting in trouble with their regulating committees, but steroids are actually one of the major types of hormones used in our body to communicate.

There are a lot of steroids in our body. Steroids come from lipids. The major lipid that these steroids come from is cholesterol.

The chemical structure of steroids is a bit like rings twisted, and chained together. These rings are made of carbon atoms.

There are:

  1. Cyclohexane rings
  2. Six-Membered carbon rings
  3. One Cyclopentane ring

This structure is a really characteristic way of signaling a cell. Unlike proteins and polypeptides, whose receptors are on the cell surface, steroids, because they’re made of lipids, have a really easy time passing through the cell membrane, and their receptors are located inside the cell.

Steroids usually go all the way inside of the cell to signal their receptor as primary messengers.

Usually these receptors can be found inside the nucleus or further back inside the cytoplasm. Steroids on the other hand can trigger effects all the way down to microscopic levels inside proteins. They trigger changes inside the cells by “transcribing” and “translating” fresh proteins.

Examples of “big steroids” in the body are those that come from the adrenal cortex, like cortisol and aldosterone, and those hormones that come from the gonads like the sex hormones testosterone and estrogen and progesterone.

Tyrosine Derivatives

Tyrosine derivatives come from the amino acid tyrosine.

What makes them really special is:

  1. They’re made of one amino acid, tyrosine
  2. Tyrosine is manipulated to make hormones
  3. Hormones are derived from tyrosine end up behaving like proteins and polypeptides and sometimes act like steroids

An example of derivatives of tyrosine in the body includes the thyroid gland like T3 and T4, like triiodothyronine and thyroxine, stimulate metabolism.

The tyrosine derivatives behave similar to steroids. A further example of a type of tyrosine derivative includes catecholamines.

Catecholamines are hormones formed inside the adrenal medulla that and are concerned with survival instincts such as fight or flight, much like epinephrine and norepinephrine.

These tyrosine derivatives act really similarly to peptides by binding on the outside of the cell and discharging secondary messengers on the inside.

Certain thyroid hormones, especially tyrosine derivatives behave like steroids. The catecholamine tyrosine derivatives behave like proteins and polypeptides.

They form their own unique class because they originate from the amino acid, tyrosine.

Remember, the structure of these hormones dictates almost everything we think or do from fear to hunger to urinating and pushing babies out.