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Principles of Endocrinology

Robert Kemppainen, DVM, PhD

Department of Anatomy, Physiology and Pharmacology, Endocrine Diagnostic Service, Auburn University, Auburn, AL, USA

Overview of Endocrinology Protein/Polypeptide Hormones

Hormones in this class are composed at minimum of
The classic definition of an endocrine tissue is a ductless three amino acids. Some consist of subunits (e.g., the
gland specialized to release a hormone into the blood­ pituitary hormones luteinizing hormone [LH], follicle‐
stream to act on a distant target. Glands such as the stimulating hormone [FSH], and thyroid‐stimulating
pituitary, thyroid, islet cells in the pancreas, and adrenal hormone [TSH]) and may have additional modifications
fit this characterization. It is increasingly clear, however, including additions of sugar residues (glycosylation),
that many (if not all) tissues in the body release sub­ phosphates, and disulfide bonds.
stances into the blood (or at least locally) in a regulated Protein/polypeptides hormones are manufactured
fashion and these substances influence the activity of by classic transcription/translation of genes that are
other cells. Examples include leptin and adiponectin expressed in high amounts in the endocrine tissue of
from adipose tissue, natriuretic factors from the heart, origin. Encoded in the genes are sequences that direct
and erythropoietin from the kidney. these molecules to the regulated secretory pathway,
Hormones regulate a vast array of biologic processes, resulting in the storage of the hormone in vesicles, ready
including, but not limited to, fuel mobilization, storage, and for release by exocytosis in response to the appropriate
distribution; maintenance of electrolyte concentrations in signal. The nature of this signal differs depending on the
extracellular fluid; and control of metabolic rate. Each system involved. Most endocrine cells that produce and
system employs checks and balances; most utilize some release hormones of this class store significant amounts
form of negative feedback control to maintain homeostasis. of the hormone and have the ability to release a large
While some systems have as their principal goal mainte­ amount of product if needed. Continual stimulation of
nance of a steady level of a metabolite or electrolyte, others release often causes induction of the gene necessary for
employ moving set points or thresholds that adapt to chang­ its synthesis.
ing conditions. As an example of the latter, stress causes Protein/polypeptide hormones are released into cir­
activation of the pituitary‐adrenal system and increased culation through the specialized capillaries in their
adrenocorticotropic hormone (ACTH) release stimulates tissue of origin and most circulate free or unbound in
release of cortisol from the adrenal cortex. Normally, blood. Exceptions exist; for example, insulin‐like
elevated cortisol concentrations suppress ACTH secretion growth factor‐1 (IGF‐1) and growth hormone are
via negative feedback, but in some long‐term stressful bound to varying extent to plasma proteins. In the case
situations, this negative feedback is overridden so that high of IGF‐1, the binding is significant, accounting for the
ACTH concentrations in circulation are maintained in prolonged half‐life of this hormone. By contrast, most
the presence of the steroid negative feedback signal. other protein/polypeptide hormones have relatively
short half‐lives, usually in the range of several minutes
Chemical Classes of Hormones (e.g., the half‐life of ACTH is less than 15 minutes).
Protein/polypeptide hormones act on target cells by
Classic hormones comprise three main chemical types. binding receptors located on the cell surface (see later),
Each class has a different means of synthesis, storage, rapidly activating specific signal transduction systems
and mechanism of action on targets. in target cells.

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical