Hormones are divided into three chemical groups

1. Peptide and protein hormones make up the majority of hormones. A peptide consists of 2 or more amino acids, and a protein is defined as having 50 or more amino acids. Insulin is therefore a small protein hormone (Figure 40.2A). Peptide and protein hormones are water-soluble and thus easily transported in the blood. They can be packaged in vesicles within the cells that make them, and then released by exocytosis.

2. Steroid hormones (such as estrogen and testosterone) are synthesized from cholesterol (Figure 40.2B), are lipid-soluble, and pass easily through cell membranes. Steroid hormones diffuse out of the cells that make them and are usually bound to carrier molecules in the blood.

3. Amine hormones are mostly synthesized from the amino acid tyrosine (thyroxine is one example; Figure 40.2C). Some amine hormones are water-soluble and others are lipid-soluble; their modes of release differ accordingly.

Figure 40.2 Three Classes of Hormones (A) The largest hormone molecules are peptides and proteins. An example of a protein hormone is the pancreatic hormone insulin. (B) Steroid hormones are modified from cholesterol molecules. They include the corticosteroids produced by the adrenal gland and the sex steroids produced primarily by the gonads. (C) Amine hormones are tiny molecules synthesized from a single amino acid. Both epinephrine and thyroxine are synthesized from tyrosine units, but thyroxine is lipid-soluble and epinephrine is water-soluble. Their modes of release and transport and the locations of their receptors differ accordingly.