The relationship between serum levels of interleukin-6 and thyroid hormone during the follow-up study in children with nonthyroidal illness: marked inverse correlation in Kawasaki and infectious disease.

Our previous study demonstrated that the serum IL-6 correlated inversely with serum T3 and the T3/T4 ratio in children with acute respiratory infection during the acute phase of illness. To investigate whether serum IL-6 inversely correlates with serum thyroid hormone during not only the active stage of the disease but also in the follow-up period in nonthyroidal illness, we measured serum levels of IL-6, T3 and T4 in 31 children from the acute to the convalescent phase. They were divided into 3 groups; 7 patients with Kawasaki disease, 16 patients with infectious disease and 8 patients with non-inflammatory disease. In the follow-up of patients with Kawasaki disease, a marked inverse relationship was observed between serum IL-6 and T3 (r = -0.844, P < 0.001) or the T3/T4 ratio (r = -0.863, P < 0.001). Serum T4 showed a weak but significant negative correlation with serum IL-6 (r = -0.474, P = 0.035) only in this situation. There was also a significant negative correlation between serum IL-6 and T3 (r = -0.582, P < 0.001) or the T3/T4 ratio (r = -0.660, P < 0.001) during the follow-up of children with infectious disease. In the follow-up study of patients with non-inflammatory disease, however, no significant relationships were observed between serum thyroid hormones and IL-6. IL-6 may be one important factor involved in the decrease in the serum T3 level and the T3/T4 ratio in patients with nonthyroidal illness particularly characterized by strong inflammation and activation of the immune system as observed in Kawasaki and infectious disease.