CONTINUOUS INFUSION OF INTERLEUKIN-1-BETA INDUCES A NONTHYROIDAL ILLNESS SYNDROME IN THE RAT

We studied the effects of continuous administration of recombinant human interleukin-1beta (IL-1) on pituitary-thyroid function. Rats were equipped with minipumps loaded with either IL-1 (delivery rate, 0.5, 2.0, or 4.0 mug/day, ip, for 1 week) or saline. Infusion of 2.0 and 4.0 mug IL-1/day caused a significant decrease in plasma free T4 levels during the first 2-4 days, whereas plasma total T4 levels and T4 binding were significantly lowered throughout the week of the study. The infusion of 0.5 mug IL-1/day did not significantly change plasma TSH or total and free T4 levels. During the infusion of 2.0 mug IL-1/day, the decrease in plasma free T4 levels was paralleled by a significant decline in plasma TSH values and an impaired TSH responsiveness to TRH administration on the second day of infusion. IL-1 (2.0 mug/day) treatment significantly lowered plasma levels of T4-binding prealbumin, whereas it did not influence the plasma T3/T4 ratio or hepatic 5'-deiodinase activity. Plasma rT3 levels remained undetectable in both control and IL-1-treated rats. Chronic infusion of rats with 4.0 mug IL-1/day induced prolonged fever, whereas at the lower doses of IL-1, temperatures were elevated only on the first 2 days. IL-1 at doses of 2.0 and 4.0 mug/day induced a transient decrease in food intake and a suppression of body weight gain. Restriction of food consumption to the level observed in the 2.0 mug IL-1 experiment caused small decreases in T3, total and free T4, and TSH levels compared to those in ad libitum fed rats, but had no effects on T4 binding. We conclude that 1) continuous infusion of rats with 2.0 and 4.0 mug IL-1/day induces changes in thyroid economy commonly seen during infectious diseases and other systemic illnesses in rats [decreased plasma levels of TSH, T3, and (free) T4; diminished T4 binding, and decreased plasma T4-binding prealbumin levels], 2) the decrease in food intake during IL-1 treatment cannot completely explain the observed changes in thyroid hormone and TSH levels; and 3) it is highly unlikely that the decrease in thyroid hormone binding during chronic IL-1 infusion is caused by decreased food intake. Further studies are needed to clarify whether the observed alterations in thyroid economy during IL-1 infusion reflect direct effects of IL-1 per se or indirect effects caused by the mild illness induced by the cytokine.