Calcium homeostasis: reassessment of the actions of parathyroid hormone.

The purpose of this report is to adjust our interpretation of the actions of parathyroid hormone (PTH) to include its action on the bone mineral-noncollagenous protein interactions at all bone surfaces. The three primary areas that respond to PTH are: (1) all bone surface areas in contact with the extracellular fluid (ECF), (2) the kidney, and indirectly the intestinal tract, and (3) the bone remodeling sequence. The primary rapid action of the hormone is to set and maintain the free calcium concentration of the ECF.