ZINC-DEFICIENCY AND THE CONCENTRATIONS OF THYROID-HORMONES IN SERUM OF FORCE-FED RATS

This study was performed to investigate the effect of zinc deficiency on thyroid-hormone metabolism. Therefore, two experiments with growing male Sprague-Dawley rats were carried out. In both experiments, all t he rats were fed sufficient quantities of diet by gastric tube in orde r to avoid the confounding effects of low food intake in zinc-deficien t individuals. In the first experiment, the zinc concentration of the diet (0.5 mg Zn/kg vs 30 mg Zn/kg) and the dietary fat (coconut oil/sa fflower oil mixture, 7:1 vs linseed oil) were varied. In the second ex periment, rats were fed zinc-adequate (40 mg Zn/kg) or zinc-deficient (0.5 mg Zn/kg) fat-free diets for 6 days. After that period, the group s were divided; half the rats were given the fat-free zinc-adequate an d zinc-deficient diets for another 3.5 days, whereas the other half wa s given the same diets supplemented with 5 % safflower oil. In both ex periments, the rats fed the zinc-deficient diets showed reduced body-w eight gain and markedly reduced activity of alkaline phosphatase and z inc concentration in serum, proving their severe zinc-deficient states . In both experiments, zinc deficiency caused hypothyroidism, characte rized by reduced concentrations of triiodo-thyronine (T-3) and total a nd free thyroxine (T-4) in serum, regardless of the dietary fat. Howev er, in the first experiment the reduction of T-3 concentrations by zin c deficiency was more pronounced in the rats fed the diet with predomi nantly coconut oil than in the rats fed the diet with linseed oil, mea ning that the type of dietary fat might influence the effect of zinc d eficiency on thyroid-hormone metabolism. The activity of hepatic thyro xine 5' monodeiodinase (5'D) determined in the second experiment was m arkedly elevated by zinc deficiency, regardless of the dietary fat lev el. The dietary fat also influenced the concentration of thyroid hormo nes. In the first experiment, the rats fed the diet with predominantly coconut oil had higher concentrations of T-3 and lower concentrations of T-4 than the rats fed the diet with linseed oil, whereas there was no difference in the concentration of free T-4 between both groups. I n the second experiment, supplementation of 5% safflower to the fat-fr ee diet increased the concentration of T-3, whereas the concentrations of total and free T-4 remained unchanged by safflower-oil supplementa tion. In conclusion, this study shows that both the zinc supply and th e dietary fat influence thyroid-hormone metabolism. This might be of p hysiological significance, because thyroid hormones mediate important physiological processes.