Effects of varying dietary iodine supplementation levels as iodide or iodate on thyroid status as well as mRNA expression and enzyme activity of antioxidative enzymes in tissues of grower/finisher pigs

Purpose: The objective of this study was to investigate the influence of high dietary iodine supply and different iodine sources on thyroid status and oxidative stress in target tissues of the thyroid hormones in fattening pigs. Methods: Eighty castrates (body weight: 33.3 ± 0.4 kg) were randomly allotted into five different treatments: The control diet contained 150 μg I/kg as KI, the other feeding groups were supplemented with 4,000 μg I/kg (as KI and KIO₃) and 10,000 μg I/kg (as KI and KIO₃), respectively. The mRNA expression levels of sodium/iodide symporter (NIS) and key antioxidant enzymes (Cu/Zn SOD, CAT, GPx) were analyzed in thyroid gland, liver, kidney, muscle, and adipose tissue sampled during slaughter. Furthermore, antioxidant enzyme activities and the effect on lipid peroxidation (MDA) were determined in liver and muscle. Results: In thyroid gland, a significant downregulation of NIS and Cu/Zn SOD mRNA expression was observed in high-iodine groups. In liver, a source effect on the mRNA expression of Cu/Zn SOD between KI and KIO₃ at 4,000 μg I/kg was shown. In contrast, not SOD but GPx activity was affected by iodine source with strongest downregulation in high KIO₃ group. In muscle, GPx activity was affected by both iodine source and dose, showing stronger downregulation in KI groups. In kidney and adipose tissue, oxidative stress parameters showed no or only unsystematic changes. However, variation in iodine supply had no effect on MDA concentrations. Conclusions: NIS expression was significantly decreased with increased iodine supplementation, which is to ensure the thyroid gland function. However, the alleviating effect of iodine supplementation observed in antioxidant enzyme mRNA expression and activity did not reflect on the lipid peroxide level.