Serum interleukin-6 and thyroid hormones in rheumatoid arthritis

Using rheumatoid arthritis (RA) as a model, we have investigated whether the activation of the cytokine system, in particular, activation of interleukin (IL)-6 production, is a major cause of the depressed serum T(3) seen frequently in the nonthyroidal illness syndrome (NTIS). RA was chosen because it is a chronic autoimmune disease leading to increased serum IL-6 concentrations. We studied 16 untreated RA and 35 treated RA patients. Twenty-seven treated and 27 untreated patients with noninflammatory musculoskeletal symptoms served as controls. The patient groups displayed similar age distribution and nutritional status. Untreated RA patients displayed elevations of serum IL-6 (mean, 37.5 pg/mL) and C-reactive protein (CRP; mean, 41.3 mg/L), consistent with the inflammatory nature of their disease. Treated RA patients had significantly reduced serum IL-6 (mean, 9.9 pg/mL) and CRP (mean, 13.3 mg/L) compared with untreated RA patients, while untreated and treated patients with noninflammatory musculoskeletal symptoms had near normal serum IL-6 (mean, 2.5, 6.6 pg/mL, respectively) and CRP levels (mean, 5.8, 8.1 mg/L, respectively). However, there were no significant differences in serum concentrations of free T(3) (FT(3)) and free T(4) (FT(4)) between groups, and thyroid indices were in the normal range in RA patients. Moreover, no significant correlations between serum concentration of IL-6 and any of the thyroid hormones were demonstrated for any of the patient groups. In conclusion, we have been unable to confirm in RA that IL-6 activation leads to the low T(3) state of NTIS.