Altered vitamin D metabolism and bone remodelling in patients with medullary thyroid carcinoma and hypercalcitoninemia

Fourteen patients with medullary carcinoma of the thyroid (MCT) and hypercalcitoninemia were studied. Serum concentrations of calcium, phosphorus and iPTH (C-terminal) were normal. Serum 1,25-dihydroxyvitamin D (1,25-(OH) 2D) levels were increased (p < 0.001) in spite of reduced serum 25-hydroxyvitamin D (25-OHD) levels (p < 0.02) indicating an enhanced activity of the renal 1 α-hydroxylase. Serum 24,25-dihydroxyvitamin D levels were normal and correlated positively with serum 25-OHD. Histomorphometric analyses of iliac crest bone biopsies after in vivo tetracycline double-labelling were performed in patients and controls. The patients showed a normal trabecular bone volume. The mean size of the cortical osteocytic lacunae was increased (p < 0.001). Significant increases were found in fractional formation surfaces (p < 0.05), fractional labelled surfaces (p < 0.01) and fractional resorption surfaces (p < 0.005) in trabecular bone. The appositional rate of newly mineralized bone was reduced (p < 0.025). The mean osteoid seam width was normal due to an unchanged mineralization lag time and a normal osteoid appositional rate. The bone formation rate at tissue level was high normal. The altered vitamin D metabolism may be caused by a direct effect of hypercalcitoninemia on the renal lα-hydroxylase or may represent an adaptive change in calcium-phosphorus homeostasis. The dynamic bone changes are similar to those found in primary hyperparathyroidism and may be caused by an enhanced sensitivity to circulating PTH induced by the increased 1,25-(OH) 2D. L'étude a porté sur 14 malades atteints de cancer médullaire du corps thyroïde avec hypercalcitoninémie. Chez ces malades, les taux sériques du calcium, du phosphore et de l'iPTH (C-terminal) étaient normaux. Le taux sérique de la 1,25 dihydroxyvitamin D (1,25-(OH) 2D) était augmenté (p < 0,001), alors qua celui de la 25-hydroxyvitamine D (25 OHD) était diminué (p < 0,02), ceci indiquant une activité accrue de la 1 hydroxylase rénale. Les taux sériques du 24–25 (OH) 2D étaient normaux et corrélés positivement avec ceux du 25 OHD. Une étude histomorphométrique de la biopsie osseuse iliaque aprés marquage par la tétracycline in vivo a été réalisée chez les malades et chez des témoins. Lee malades avaient un volume trabéculaire osseux normal. La surface moyenne des lacunes périostéocytaires de l'os cortical était augmentée (p < 0,001). On notait aussi une augmentation significative du pourcentage des surfaces de forma