[The mechanism of bone loss in rheumatoid arthritis. Description based on a case report].

Various factors influencing bone turnover and bone loss in rheumatoid arthritis (RA) are illustrated using the example of a postmenopausal woman with a highly active RA. In particular, the relationships between disease activity, vitamin D metabolism, parathyroid hormone (PTH) levels and calcium metabolism are described. High disease activity is associated with low levels of 25-hydroxycholecalciferol, and especially of 1.25-dihydroxycholecalciferol. Despite vitamin D deficiency, PTH levels were decreased and histomorphometric investigation of the iliac crest biopsy showed severe osteoporosis but no signs of osteomalacia. Suppression of the inflammatory disease activity of RA led to a normalisation of the serum levels of 1.25-dihydroxycholecalciferol and PTH. This was associated with a reduction in the initially increased levels of bone specific alkaline phosphatase to normal values. This case report shows a close relationship between disease activity and bone turnover in RA and indicates that early investigation and therapy of disturbances of bone metabolism in RA are necessary.