The effect of iodine or selenium and iodine on thyroid hormone production under selenium deficiency conditions in Farahani sheep

Background and objectives: Iodine is a structural component of thyroid hormones (T3 and T4), which are essential for the life of mammals. Iodine deficiency leads to insufficient production of thyroid hormones, which is primarily due to the lack of iodine in the soil. Selenium is also involved in the metabolism of thyroid hormones by participating in the construction of selenoproteins that convert the inactive form of T4 into the active form of T3. The aim of this study was to investigate the effects of iodine supplementation on thyroid hormone concentrations of in ewes exposed to iodine deficiency under selenium deficient or sufficient conditions.
Materials and methods: A flock of sheep was selected in an area where selenium and iodine deficiency had been identified. In this flock, 30 young ewes (2-3 births) of Farahani breed with similar body weight (47 ± 6 kg) were randomly divided into three groups, including a control group and two experimental groups. The first and second experimental groups received 390 mg of iodine in the form of fatty acid ethyl ester from safflower oil at the end of gestation at the end of winter season, and the second experimental group received 10 ml of selenium supplement in the form of subcutaneous injection. The concentration of thyroid hormones, including triiodothyronine and thyroxine, inorganic iodine in serum and selenium concentration in blood serum were measured and compared before the beginning of the experiment and monthly until the third month in the experimental groups.
Results: The inorganic iodine concentration in the serum of the experimental animals at the beginning of the experiment was 15.53 μg /L and the selenium concentration in the blood serum was 79.73 ng/ml, both of which were in the deficiency range. Iodine supplementation caused a significant increase in the concentration of inorganic iodine in blood serum in the experimental groups compared to the control group in all three months of sampling (P < 0.01). Selenium supplementation also caused a significant increase in blood serum selenium concentration in the selenium+ iodine-supplemented group compared with baseline and the control and iodine supplemented groups in all months of the experiment (P < 0.01). In the first experimental month after iodine supplementation, an increase in T4 hormone concentration was observed in the iodine-supplemented group and in the iodine + selenium supplemented group compared with baseline and the control group. The concentration of T4 hormone was higher in the iodine supplementation group than in the control group and in the iodine + selenium supplementation group in the second and third months. Iodine supplementation increased T3 concentration in the first and second months of the trial compared with baseline, and selenium supplementation also significantly increased T3 concentration in the first and second months of the trial in the iodine + selenium supplemented group compared with the iodine supplemented group and the control group. The increase in the concentration of thyroid hormones after iodine supplementation showed that the production of these hormones was affected by iodine deficiency. Selenium supplementation increased the concentration of T3 in the first and second months and decreased the ratio of T4 to T3 in the second month. It showed that the effect of iodine supplementation is better in the conditions of selenium sufficiency.
Conclusion: The results indicate the effect of iodine supplementation to increase inorganic iodine concentration in serum, and the increase in thyroid hormone concentration after iodine supplementation indicates the limiting effect of iodine deficiency on the production of these hormones under iodine deficiency conditions. In selenium deficiency, the effect of iodine supplementation on T3 hormone production may be limited.