Effects of a dietary thermally oxidized fat on thyroid morphology and mRNA concentrations of thyroidal iodide transporter and thyroid peroxidase in rats

Abstract

Background/aim: Recent studies demonstrated that feeding oxidized fats increases the concentrations of total and free thyroxine in blood of rats and pigs. This finding suggested that oxidized fats affect the function of the thyroid gland. This study investigates the effects of a thermally oxidized dietary fat on the morphology of the thyroid gland and on the expression of proteins (Na(+)/I(-) symporter, thyroid peroxidase) involved in the synthesis of thyroid hormones in rats at different dietary iodine concentrations.

Methods: An experiment was conducted with 48 growing male Sprague-Dawley rats which were allotted to four groups of 12 animals each. According to a bifactorial experimental design, the rats received semisynthetic diets with 10% of either a fresh or an oxidized fat, with low (50 microg/kg) or adequate (400 microg/kg) iodine concentrations, over a period of 38 days. The oxidized fat was prepared by heating sunflower oil at a temperature of 55 degrees C for a period of 5 weeks. The oxidized fat had much higher concentrations of lipid peroxidation products than the fresh fat as assessed by determining the peroxide concentrations (877 vs. 33 mEq O(2)/kg) and those of thiobarbituric acid reactive substances (25 vs. 0.7 micromol/kg).

Results: Rats fed the diets containing oxidized fat had higher concentrations of total and free thyroxine in plasma, a greater height of thyroid epithelial cells, a smaller diameter of thyroid follicle lumen, a lower Na(+)/I(-) symporter mRNA concentration, and a higher thyroid peroxidase mRNA concentration than rats fed the fresh fat (p < 0.05 for all effects). The concentrations of triiodothyronine and thyrotropin were not different between rats fed the fresh fat and those fed the oxidized fat. The dietary iodine supply also had significant effects on some of the parameters analyzed. There were no interactions between type of fat and dietary iodine concentrations.

Conclusion: The rat model used here shows that dietary oxidized fats affect the morphology and the function of the thyroid gland, irrespective of the dietary iodine supply.