Serum levels of vitamin D metabolites and bone remodelling in hyperthyroidism

Serum levels of 25-hydroxyvitamin DF, 24,25-dihydroxyvitamin D and 1,25-dihydroxyvitamin D were measured in 25 untreated hyperthyroid patients in whom histomorphometric evaluations of iliac crest bone biopsies were performed after in vivo tetracycline doublelabeling. The serum concentration of 25-hydroxyvitamin D was normal. The serum concentration of 1,25-dihydroxyvitamin D was reduced (p less than 0.02) compared to normal whereas the serum concentration of 24,25-dihydroxyvitamin D was increased (p less than 0.02). The bone changes were characterized by an enhanced turn-over in trabecular and cortical bone leading to an increased porosity of cortical bone and mobilisation of bone mineral. The observed changes in vitamin D metabolism could be explained by a reduced renal 1-alpha-hydroxylase activity secondary to hypercalcaemia with suppressed parathyroid secretion and hyperphosphataemia. The bone changes were unrelated to the serum levels of vitamin D metabolites. In trabecular bone the appositional rate and mineralization rates of osteoid were increased and the mineralization lag time was shortened showing that the mineralization and formation of osteoid in the hyperthyroid state can progress with an enhanced rate in spite of a reduced mean serum level of the active vitamin D metabolite, 1,25-dihydroxyvitamin D.