Normal circulating triiodothyronine concentrations are maintained despite severe hypothyroidism in growing pigs fed...

Experiments were designed to study effects of dietary rapeseed presscake meal on the thyroid and on the liver 5'-monodeiodinase activity in growing pigs. Animals were fed rapeseed presscake meal (15% in the ration) of 0-varieties (containing relatively high amounts of glucosinolates and goitrin) or a control diet (soybean meal instead of rapeseed presscake meal) without or with thyroxine added to feed. Food intake and average daily gain were comparable because pigs were pair-fed. Serum thiocyanate concentration was significantly greater in pigs fed rapeseed presscake meal. Pigs fed rapeseed presscake meal developed hypothyroidism. Serum free thyroxine concentrations in rapeseed presscake meal-fed pigs were significantly lower than in controls, normal in thyroxine-supplemented pigs fed rapeseed presscake meal and significantly above normal in thyroxine-supplemented controls. Serum free triiodothyronine concentrations were not significantly influenced by rapeseed presscake meal feeding or thyroxine supplementation. Liver weight and total DNA content in pigs fed rapeseed presscake meal were higher than in controls but were not significantly affected by thyroxine feeding. Hepatic 5'-monodeiodinase activity on a protein basis was lower in pigs fed rapeseed presscake meal and was not normalized by thyroxine supplementation. However, in whole liver, because of greater liver mass, 5'-monodeiodinase activity in pigs fed rapeseed presscake meal was similar to that in controls. The data indicate that serum free triiodothyronine concentrations in pigs fed rapeseed presscake meal could be maintained in the physiological range, probably because of enhanced triiodothyronine secretion and sufficient extrathyroidal thyroxine to triiodothyronine conversion.