INCREASED SERUM INTERLEUKIN-6 CONCENTRATION IN PATIENTS WITH SUBACUTE THYROIDITIS - RELATIONSHIP WITH CONCOMITANT CHANGES IN SERUM T4-BINDING GLOBULIN CONCENTRATION

Interleukin-6 (IL-6) is the main mediator of the acute phase response. Increased serum concentrations of the cytokine have been found in patients with nonthyroidal inflammatory disorders and infections. In 18 patients with subacute thyroiditis (SAT) evaluated within 1-2 weeks after the onset of the disease, serum IL-6 values, as assessed by an ELISA method having a limit of detection of 25 fmol/L, ranged 139.2-543.9 fmol/L (mean+/-SE, 287.2+/-28.2 fmol/L). These values were significantly higher than those of 25 normal healthy controls (mean+/-SE, 26.2+/-5.5 fmol/L, range <25-99.4), 18 of whom had serum IL-6 values below the detection limit. The increase in serum IL-6 levels in SAT patients appeared to be related to the inflammatory disorder and not to thyrotoxicosis, because 18 Graves' disease patients and 13 patients with toxic adenoma or toxic multinodular goiter had significantly lower serum IL-6 concentrations (101.7+/-35.2 fmol/L. range <25-251, for Graves' disease, 79.6+/-41.4 fmol/L, range <25-168.5, for toxic adenoma, p<0.001 vs SAT for both groups) despite the markedly higher levels of total and free thyroid hormones. Neither free T4 nor free T3 values were correlated with serum IL-6 levels both in SAT and Graves' patients. Twelve SAT patients were reevaluated 3-4 months later, after remission of the disease and at least one month after glucocorticoid withdrawal. At the final observation, all SAT patients showed a normalization of IL-6 concentration, which was undetectable in 8/12 (mean+/-SE, 22.8+/-5.4 fmol/L, p<0.001 vs acute phase values). Mean serum TBG concentration was reduced in SAT patients at the time of first observation (19.3+/-1.1 mg/L, p<0.001 vs controls). While no correlation could be found between serum TBG and serum total or free thyroid hormone levels, serum TBG concentration was inversely correlated with serum IL-6 concentration (r=0.69, p<0.01). Remission of the disease was associated with a significant increase in mean serum TBG concentration to 24.3+/-1.1 mg/L (p<0.001 vs acute phase values). In conclusion, SAT is associated with an increased serum concentration of IL-6, which normalizes upon remission of the disease. The rise in serum IL-6 levels may at least in part account for the concomitant decrease in serum TBG concentration.