NATURE OF HORMONES

Hormones can be divided into five major classes: 

(1) amino acid derivatives such as dopamine, catecholamine, and thyroid hormone.

(2) small neuropeptides such as gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), somatostatin, and vasopressin

(3) large proteins such as insulin, luteinizing hormone (LH), and PTH produced by classic endocrine glands

(4) steroid hormones such as cortisol and estrogen that are synthesized from cholesterol-based precursors.

(5) vitamin derivatives such as retinoids (vitamin A) and vitamin D. 

A variety of peptide growth factors, most of which act locally, share actions with hormones. As a rule, 

amino acid derivatives and peptide hormones interact with cell-surface membrane receptors. 

Steroids, thyroid hormones, vitamin D, and retinoids are lipid-soluble and interact with intracellular nuclear receptors.