Characteristics of secondary hyperparathyroidism in vitamin D-deficient dogs

Characteristics of secondary hyperparathyroidism were evaluated in dogs with mild vitamin D deficiency. The animals were normocalcemic with reduced concentrations of 1,25-dihydroxy vitamin D3 [1,25(OH)2D3] and elevations in parathyroid hormone (PTH) concentrations, parathyroid mass, and prepro PTH mRNA levels. Dynamic testing revealed a sigmoidal relationship between plasma calcium and PTH, although PTH concentrations were increased relative to values in vitamin D-sufficient dogs. Infusions of chelator elicited lower plasma calcium levels and greater augmentations in biologically active PTH in vitamin D-deficient than in D-sufficient animals. Induced hypercalcemia lowered both immunoreactive and bioactive hormone to stable but detectable levels. The results demonstrate the decreased capacity of vitamin D-deficient animals to defend against acute hypocalcemia, despite the presence of abundant PTH, and indicate that increased circulating PTH levels in early vitamin D deficiency is due predominantly to an augmentation in the quantity of releasable hormone. The latter appears secondary to an increase in parathyroid mass and synthetic activity regulated by 1,25(OH)2D3 per se.