Many patients with systemic lupus erythematosus (SLE) and fibromyalgia (FM) may spend less time exposed to the sun than healthy individuals and thus might have low vitamin D levels. It is known that hydroxychloroquine (HCQ) inhibits conversion of 25(OH)- to 1,25(OH)2-vitamin D both in vitro and in patients with sarcoidosis. We assessed winter serum 25(OH)- and 1,25(OH)2-vitamin D levels in patients with SLE and FM.
We recruited 25 consecutive female SLE and 25 female FM patients in London, Ontario, between January and March 2000. Subjects completed a brief questionnaire. Serum levels of 25(OH)-, 1,25(OH)2-vitamin D, and parathyroid hormone (PTH) were measured.
In SLE patients mean 25(OH)-vitamin D was 46.5 nmol/l and mean 1,25(OH)2-vitamin D was 74.4 pmol/l. In FM patients these means were 51.5 nmol/l and 90.1 pmol/l, respectively. Serum 25(OH)-vitamin D levels did not significantly differ between SLE and FM patients, nor after adjusting for age and vitamin D, milk consumption, and sun block use. In 14 of the SLE patients and 12 of the FM patients 25(OH)-vitamin D levels < 50 nmol/l were found. SLE patients not using vitamin D supplements had lower 25(OH)-vitamin D levels than those who did. 1,25(OH)2-vitamin D tended to be lower in the SLE compared to the FM patients. This difference could be attributed to HCQ use: HCQ users (n = 17) had lower 1,25(OH)2-vitamin D levels than nonusers (n = 33); the mean adjusted difference was 24.4 pmol/l (95% CI 2.8-49.9).
Half the SLE and FM patients had 25(OH)-vitamin D levels < 50 nmol/l, a level at which PTH stimulation occurs. Our data suggest that in SLE patients HCQ might inhibit conversion of 25(OH)-vitamin D to 1,25(OH)2-vitamin D.