[1] Department of General Practice, University of Oslo
[2] Oslo Sanitetsforening Rheumatism Hospital, Oslo 1, N-0172
[3] Norway and Oslo Sanitetsforening Rheumatism Hospital, Oslo
[4] University Department of Growth and Reproduction, Rigshospitalet, Copenhagen
[5] Department of Clinical Chemistry, Ullevaal University Hospital, Oslo
[6] Department of Physiology and Nutrition, Norwegian College of Veterinary Medicine, Oslo