The sick euthyroid syndrome is a state of altered thyroid hormone metabolism which occurs during illness. The pathogenesis is incompletely understood but recent studies indicate a role of cytokines. It is unknown if cytokines released during illness are directly responsible for the changes in thyroid hormone metabolism. Therefore we studied if previous immunoneutralization of cytokines can prevent endotoxin (lipopolysaccharide LPS), induced sick euthyroid syndrome.
LPS administration resulted in systemic illness, an increase in serum tumor necrosis factor (TNFα) and interleukin (IL)-6 and a decrease in serum triiodothyronine (T3) and thyroxine (T4). Immunoneutralization of the effects of cytokines was accomplished by administration of monoclonal antibodies against mouse IL-1 type-1 receptor (IL-1R), TNFα, IL-6 or interferon (IFNα) prior to LPS. The LPS-induced release of cytokines was affected by previous immunoneutralization as compared with control experiments with normal immunoglobulin (IgG): anti-IL-1R did not affect serum TNFα but decreased serum IL-6, anti-TNFα decreased serum TNFα but not IL-6, anti-IL-6 did not affect serum TNFα but hugely increased IL-6 and anti-IFNγ decreased both serum TNFα and IL-6. Specific immunoneutralization of IL-1, TNFα or IFNγ did not prevent the LPS-induced decrease in serum T3, T4 and liver 5′-deiodinase mRNA. However, immunoneutralization of IL-6, although not preventing the fall in serum T3 and T4, did mitigate the LPS-induced decrease in liver 5′-deiodinase mRNA.
In view of possible non-specific effects of the huge dose of immunoglobulins (1 mg), used only in the immunoneutralization of IL-6, we repeated the experiment with F(ab′)2 fragments of anti-IL-6 antibodies. Compared with F(ab′)2 fragments of control IgG, anti-IL-6 F(ab′)2 did not affect the LPS-induced rise in serum TNFα or the decrease in serum T3 and T4 and liver 5′-deiodinase mRNA. Serum IL-6 levels induced by LPS were, however, cleared more rapidly from the circulation when anti-IL-6 F(ab′)2 fragments rather than intact anti-IL-6 were administered. In conclusion, immunoneutralization of IL-1, TNFα or IFNγ did not prevent the LPS-induced sick euthyroid syndrome in mice; immunoneutralization of IL-6, however, transiently inhibits the LPS-induced decrease of liver 5′-deiodinase mRNA.
Journal of Endocrinology (1997) 153, 115–122