In Animals, a chemical secreted by an endocrine gland whose products are released into the circulating fluid. Plant hormones or synthetic hormones which are used only to alter or control various physiologic processes, e.g., reproductive control agents, are assigned to the Class BiologicallyActiveSubstance. Hormones act as chemical messengers and regulate various physiologic processes such as growth, reproduction, metabolism, etc. They usually fall into two broad categories, viz. steroid hormones and peptide hormones.