Parathyroid Glands and Bone

Issue: Supplement to issue 3/2017

Author: Boyanov, Mihail А.

Clinic of Endocrinology and Metabolic Diseases, Department of Internal Medicine, “Alexandrovska” University Hospital, Sofia Medical University

Abstract:
PTH, an 84-amino acid peptide hormone syn-thesized by the parathyroid glands, is essential for the maintenance of calcium homeostasis. While in its traditional metabolic role, PTH helps to main-tain the serum calcium concentration within nar-row, normal limits and participates as a determi-nant of bone remodeling, more specific actions, described as catabolic and anabolic, are also well known. Clinically, the catabolic effect of PTH is best represented by primary hyperparathyroidism (PHPT), while the osteoanabolic effect of PTH is best seen when PTH or its biological amino-termi-nal fragment [PTH (1-34)] is used as a therapy for osteoporosis. These dual functions of PTH are unmasked under very specific pathological (PHPT) or therapeutic conditions. At the cellular level, PTH favors bone resorption, mostly by affecting the receptor activator of nuclear factor κ-B (RANK) lig-and (RANKL)-osteoprotegerin-RANK system, lead-ing to an increase in osteoclast formation and activity. Increased bone formation due to PTH therapy is explained best by its ability to enhance osteoblastogenesis and/or osteoblast survival. The PTH-induced bone formation is mediated, in part, by a decrease in SOST/sclerostin expression in osteocytes. This review focuses on the dual ana-bolic and catabolic actions of PTH on bone, situa-tions where one is enhanced over the other, and the cellular and molecular mechanisms by which these actions are mediated.

Key words: parathyroid glands, parathormone, bone, calcium homeostasis, bone remodeling

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