TABLE 1.   Features of cytokines and classical endocrine hormones Cytokine Classical Endocrine Hormone Structure Large polypeptides, proteins, or glycoproteins Small to large polypeptides, proteins, glycoproteins, derivatives of aromatic amino acids, or steroids Cell sources Secreted by white blood cells (and many other cells in numerous types of tissues and organs) Secreted by one type of specialized cell within a ductless gland (an "endocrine gland") Concentrations in healthy, stress-free subjects Very low (virtually absent); increase markedly during tissue disease, injury, or repair Usually measurable and commonly display pulsatile and circadian patterns of secretion Location of action relative to secretion Act predominantly locally, in a paracrine or autocrine manner Act on distant target cells Range of activities "Pleiotropic," multiple target cell types and broad spectrum of actions Breadth of actions highly variable: many anterior pituitary hormones (e.g., TSH, ACTH) have highly limited actions, but "target gland" hormones (e.g., T3, T4, and glucocorticoids) have very broad range of activities "Redundancy" Display great overlap of biological activities (i.e., "redundancy") Far less overlap of biological activities; deficiency in single hormone usually produces marked abnormalities Function Function predominantly as regulators of local tissue processes Function predominantly as regulators of systemic or "whole body" homeostasis TSH, thyroid-stimulating hormone; ACTH, adrenocorticotropin; T3, 3,3',5-triiodothyronine; T4, thyroxine.