1. Cutolo M, Otsa K, Laas K, Yprus M, Lehtme R, Secchi ME et al (2006) Circannual vitamin D serum levels and disease activity in rheumatoid arthritis: Northern versus Southern Europe. Clin Exp Rheumatol 24(6):702–704
2. Frediani B, Rossini M, Adami S, Bianchi G, Di Munno O, Sinigaglia L et al (2006) Study of vitamin D status of rheumatoid arthritis patients. Rationale and design of a cross-sectional study by the osteoporosis and metabolic bone diseases study group of the Italian Society of Rheumatology (SIR). Reumatismo 58(4):314–318
3. Kröger H, Penttilä IM, Alhava EM (1993) Low serum vitamin D metabolites in women with rheumatoid arthritis. Scand J Rheumatol 22(4):172–177
4. Cantorna MT, Mahon BD (2004) Mounting evidence for vitamin D as an environmental factor affecting autoimmune disease prevalence, Exp Biol Med (Maywood) 229(11):1136–1142; 67(4):530–535. Epub 2007 Jul 31
5. Oelzner P, Müller A, Deschner F, Hüller M, Abendroth K, Hein G et al (1998) Relationship between disease activity and serum levels of vitamin D metabolites and PTH in rheumatoid arthritis. Calcif Tissue Int 62(3):193–198