THE RELATIONSHIP BETWEEN SERUM LEVELS OF INTERLEUKIN-6 AND THYROID-HORMONE IN CHILDREN WITH ACUTE RESPIRATORY-INFECTION

To determine whether monocyte-derived humoral factors such as tumor necrosis factor-alpha (TNF alpha), interleukin-1 beta (IL-1 beta), and IL-6 modulate serum thyroid hormone levels in infectious disorders, we measured serum levels of these monocyte-derived products, T-3, T-4, and C-reactive protein (CRP) in 59 out-patients with acute respiratory infection, aged 5 months to 15 yr (mean +/- SD, 5.7 +/- 4.2 yr). To minimize individual variation in their clinical and nutritional conditions, we selected out-patients with fever lasting for at least 3 days who had no severe disturbance of food intake. Serum T-3 concentrations tended to be low in these subjects; 3 (5.1%) had levels below 1.2 nmol/L, and 13 (22.0%) had levels below 1.5 nmol/L. Serum T-3 levels in 22 patients with elevated IL-6 (>15 pg/mL) were significantly lower than those in 19 patients with normal IL-6 (less than or equal to 5 pg/mL; TB, 1.7 +/- 0.4 vs. 2.2 +/- 0.6 nmol/L; P < 0.05). The serum IL-6 concentration was correlated inversely with the serum Tg level (r = -0.324; P = 0.012) as well as the T-3/T-4 ratio (r = -0.279; P = 0.032). Furthermore, a significant negative correlation was observed between CRP (induced mainly by IL-6) and T-3 (r = -0.408; P = 0.001) or the T-3/T-4 ratio (r = -0.302; P = 0.020). On the other hand, TNF alpha was measurable in only 9 of 59 patients, and there was no correlation between TNF alpha and levels of thyroid hormone, CRP, or IL-6. Serum IL-1 beta was weakly detected in only 1 patient. On follow-up study of 10 of these patients, the serum T-3 and T-3/T-4 ratio increased significantly with a reciprocal decrease in IL-6 and CRP during convalescence. No remarkable change in the serum T-4 level was observed in our study. Our data suggest that the euthyroid sick syndrome is relatively common in children with acute respiratory infection, and IL-6 appears to mediate this syndrome in infectious disorders.