Background: The inverse correlation of serum vitamin D (25 (OH) D) level and disease activity in patients with rheumatoid arthritis (RA) is not consistently documented across different ethnic groups.
Objective: To investigate the relationship of 25 (OH) D level with disease activity and functional ability in Thai patients with RA.
Material and Method: Between June, 2012 and December,2012 90 RA patients were enrolled from the outpatient department of Thammasat University Hospital, Pathumthani, Thailand. 25 (OH) D concentrations were measured and the disease activity score 28 (DAS-28) and Thai health assessment questionnaire (Thai HAQ) were assessed. Based on the DAS-28, patients were divided into three groups: (i) low <3.2, (ii) moderate >3.2 to 5.1, and (iii) severe >5.1. 25 (OH) D level were compared among groups.
Results: Of 90 patients, 20 (22.2%) and 48 (53.3%) were vitamin D deficiency (25 (OH) D <20 ng/ml) and insufficiency (25 (OH) D >20 to 30 ng/ml), respectively. The mean 25 (OH) D concentration was 25.6+6.9 ng/ml. There was no significant difference in serum vitamin D among three groups (p = 0.20). Neither disease activity nor functional ability was correlated with serum vitamin D level (p = 0.98 and 0.93 respectively).
Conclusion: Most of rheumatoid arthritis patients had inadequate serum vitamin D. There was no correlation between serum vitamin D level and disease activity and functional ability.

Keywords: Serum vitamin D, Rheumatoid arthritis, Disease activity score