Rapeseed meals, methimazole, thiocyanate and iodine affect growth and tyroid. Investigations into glucosinolate tolerance in the pig

In two factorial experiments with a total of 81 growing pigs, the effects of antithyroid compounds and rapeseed meal (RSM) diets differing in glucosinolate content were tested without or with supplementary iodine. In Experiment 1, a 4 x 3 factorial arrangement was used to investigate the effects of antithyroid compound inclusion and supplementary iodine. Pigs received a grain-soya bean meal diet (control) or this diet with 250 mg kg-1 methimazole (MMI), 1000 mg kg-1 thiocyanate (SCN- as KSCN) or 80 g kg-1 high glucosinolate RSM (9.5 mmol glucosinolates kg-1), each diet supplemented without or with iodine; 0, 125 micrograms kg-1 (requirement) or 500 micrograms kg-1. In Experiment 2, a 5 x 3 factorial arrangement was used to study the effects of dietary glucosinolate content and iodine supplementation. Pigs received a grain-soya bean meal diet or four diets with 160 g kg-1 RSM differing in glucosinolate content (0.6, 2.4, 6 or 19 mmol kg-1 diet), each diet supplemented with iodine: 62.5 micrograms kg-1, 125 micrograms kg-1 (requirement) or 250 micrograms kg-1. Irrespective of the iodine dosage, MMI suppressed production of thyroxine and resulted in goitre, myxoedema and cretinism. SCN- induced clinical hypothyroidism when there was no supplemental iodine. Despite high iodine supplementation, glucosinolate load (19 mmol kg-1 diet) decreased growth, feed intake, iodine store of the thyroid and serum concentration of thyroid hormone and resulted in goitre formation. In the case of diets with less than or equal to 6 mmol glucosinolates kg-1, iodine prevented anti-thyroid effects. However, more iodine was required by these pigs than by control animals to a high T4 serum concentration. The lowest glucosinolate level tested (0.6 mmol kg-1 diet) yielded the same thyroid hormone status and thyroid weight as the control, however, the iodine content of thyroid gland was decreased. In conclusion, iodine administration prevents the effects of a low glucosinolate intake only, by overcoming the depressed thyroxine released of the thyroid, resulting in regression of goitre.