Further studies on the interrelationship between parathyroid hormone and vitamin D.

Calcium homeostasis is achieved by the interplay of a number of humoral agents (1). Of central importance are the activities of parathyroid hormone (PTH) and of vitamin D. This was fully appreciated 35 years ago (2), but in spite of considerable effort, the nature of the relationship remains controversial. Recent opinion has varied from that of a complete dependence of hormone action upon the presence of vitamin D (3) to that of a complete independence of action (4, 5). In our initial studies of this problem, we concluded that in the absence of vitamin D a physiologic concentration of parathyroid hormone exerted its usual effect upon the renal excretion of phosphate but was unable to mobilize calcium from bone (6). Since these studies were published, two groups of investigators have challenged our conclusions (7, 4). Harrison and Harrison reaffirmed their previous observations (7) and concluded, as they had before, that all of the actions of parathyroid hormone depended upon vitamin D. At the opposite extreme, Ney and associates concluded that all of the effects of parathyroid hormone are exerted in the D-deficient organism. These differences are difficult to explain. However, both studies differed from ours in several respects. In the study of Harrison and Harrison (7), no attempt was made to define or control the level of endogenous parathyroid hormone, although we found this