Effect of varying glucosinolate and iodine intake via rapeseed meal diets on serum thyroid hormone level and total iodine in the thyroid in growing pigs.

In a trial with 50 fattening pigs (20 kg initial body weight), the effect of untreated rapeseed meal (RSM) (148 mmol glucosinolates and aglucones per kg dry matter) on the thyroid was compared with RSM treated with Cu2+ (9.5 mmol glucosinolates and aglucones per kg dry matter) and soybean meal (SBM). The diets containing 8% RSM were supplemented with 0.0625-1.0 and the SBM diet (control) with 0.125 mg iodine kg-1 (I). In comparison with SBM fed control, RSM treatment with Cu2+ resulted in a complete normalization of feed intake and growth. Only untreated RSM without I supplementation depressed performance and resulted in symptoms of I deficiency, but the thyroid and liver weight were also increased and the serum T4 content was significantly reduced in animals which were given RSM not supplemented with I, but treated with Cu2+. In young pigs (4 weeks) a plateau of the serum T4 content was achieved from 0.5 mg I kg-1 of the RSM diet onwards. In contrast, when the concentration of goitrogens was reduced by the treatment with Cu2+, the serum T4 level was increased significantly in groups fed with 0.125 mg I kg-1 diet and more. In older pigs (15 weeks) neither the content of goitrogens nor the I dosage affected the serum T4 level. On the other hand, the I content in the thyroid was a good indicator of the different goitrogenicity of the diet in the case of a low I supply. The present investigations show that pig diets with RSM (greater than 10 mmol glucosinolates and aglucones kg-1) should contain at least 0.5 mg I kg-1, but 0.1 mg supplementary I per kg is sufficient in diets without or with a low content (less than 1 mmol glucosinolates and aglucones per kg-1) of antithyroid compounds.