Hormones can be classified into following three groups on the basis of their chemical structure:

 i.       Amino acid derivatives
ii.    Peptide hormones
iii. Lipid derivatives

AMINO ACID DERIVATIVES

Amino acid derivatives are relatively small molecules that are structurally similar to amino acids, the building blocks of proteins. This group of hormones (sometimes known as the biogenic amines) includes epinephrine, norepinephrine, dopamine, the thyroid hormones, and melatonin. Epinephrine, norepinephrine and dopamine are structurally similar.

 Thyroid hormones are produced by the thyroid gland and melatonin is produced by the pineal gland. Epinephrine, norepinephrine, dopamine and thyroid hormones are synthesized from molecules of the amino acid tyrosine. Melatonin is manufactured from molecules of the amino acid tryptophan.

PEPTIDE HORMONES

 “Peptide hormones are chains of amino acids.”

 In general, peptide hormones are produced as prohormones. Prohormones are inactive molecules that are converted to active hormones either before or after they are secreted.

 Peptide hormones can be divided into following two groups:

 1.         Short Polypeptide Chains & Small Proteins
                    It is large and diverse group. It includes hormones that range from short polypeptide chains such as antidiuretic hormone (ADH) and oxytocin (9 amino acids apiece), to small proteins, such as growth hormone (GH; 191 amino acids) and prolactin (PRL; 198 amino acids).

                  This group includes all the hormones secreted by the hypothalamus, posterior pituitary gland, heart, thymus, digestive tract, and pancreas and most of the hormones of the anterior pituitary gland.

2.         Glycoproteins
                        This group of peptide hormones consists of glycoproteins. These proteins are more than 200 amino acids long and have carbohydrate side chains. The glycoproteins include thyroid stimulating hormone (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) from the anterior pituitary gland as well as several hormones produced in other organs.

LIPID DERIVATIVES

There are two classes of lipid derivatives:

 i.       Steroid hormones - derived from cholesterol
ii.    Eicosanoids - derived from arachidonic acid (a 20-carbon fatty acid)

Steroid Hormones
            Steroid hormones are lipids struc­turally similar to cholesterol. Steroid hormones are released by: 

a.      Male and female re­productive organs (androgens by the testes, estrogens and progestins by the ovaries)

b.      The adrenal glands (corticosteroids) and

c.      The kidneys (calcitriol)

                The individ­ual hormones differ in the side chains attached to the basic ring structure. In the blood, steroid hormones are bound to spe­cific transport proteins in the plasma. For this reason, they remain in circulation longer than do secreted peptide hormones. The liver gradually absorbs these steroids and converts them to a soluble form that can be excreted in the bile or urine.

Eicosanoids
                Eicosanoids are small molecules with a five-carbon ring at one end. These compounds are important paracrine factors that coordinate cellular activities and affect enzymatic processes (such as blood clotting) that occur in extracellular fluids. Some of the eicosanoids also have secondary roles as hor­mones. Examples of important eicosanoids are leukotrienes and prostaglandins.