Effect of iodine source and dose on growth and iodine content in tissue and plasma thyroid hormones in fattening pigs.

The aim of the present feeding trial with iodine was to assess pigs' growth performance and carcass characteristics, the iodine accumulation in tissues, and their influences on the thyroid hormones in plasma. Eighty pigs (33-115 kg body weight) were allotted to 5 dietary treatments: a control group (150 μg I/kg), two potassium iodide [KI] groups (4,000 and 10,000 μg I/kg), and two potassium iodate [KIO₃] groups (4,000 and 10,000 μg I/kg). Iodine concentration was determined in thyroid gland, liver, kidney, muscle, fat, and skin by ICP-MS. Furthermore, thyroxine (T₄) and triiodothyronine (T₃) in plasma were evaluated. High dietary iodine tended to have a negative effect on younger animals' growth (average daily gain, ADG). However, during the entire growth period, the growth performance and carcass characteristics were not influenced by iodine dosages or sources. Irrespective of iodine source, higher iodine doses of diets affected higher iodine stores in all tested tissues except for abdominal fat. Thus, iodine supplementation with 10,000 μg I/kg feed significantly increased iodine content in thyroid gland (+122%), liver (+260%), kidney (+522%), muscle (+131%), and skin (+321%) compared to the control group. However, there was no significance of thyroid hormones in plasma. As a result, pork and fat of pigs showed only low iodine accumulation even in the high-iodine groups. Thus, there should be no risk of an iodine excess in human nutrition and animal health, and the EU-upper level for iodine in pig feed can be maintained.