[Bone metabolism in primary hypothyroidism in the adult]

The aim of the study was to investigate bone metabolism in adult onset idiopathic myxedema. We studied 13 untreated patients (11 women and 2 men, age ranging 24-64 years) and, as a control group 15 healthy subjects (13 women and 2 men, age ranging 24-64 years). The hypothyroid group had significantly lower urinary excretion of hydroxyproline (4.40 +/- 0.63 vs. 8.60 +/- 1.3 mg/g, p less than 0.05) and serum concentration of osteocalcin (4.45 +/- 0.41 vs. 7.76 +/- 0.55 ng/ml, p less than 0.001). This low urinary excretion of hydroxyproline points to reduced osteoclastic bone resorption, while the low serum level of osteocalcin supports the view that osteoblastic bone formation is sluggish, too. Therefore bone metabolism in adult myxedema is characterized by a general reduction of remodelling. The stimulating actions of thyroid hormones on both bone resorption and new synthesis are further supported by the positive correlations between free hormone fractions and either urinary hydroxyproline and serum osteocalcin. Serum calcium, phosphorus, urinary calcium and phosphorus, and serum calcitrophic hormones (vitamin D, calcitonin, parathyroid hormone, measured as intact molecule) did not differed between the two groups; this finding might be related to the low fraction of active bone in hypothyroidism. The slow bone turnover seems to have few clinical consequences, but replacement therapy might produce accelerated osteoporosis, perhaps as a result of bone hypersensitivity to thyroid hormones.