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Normal Circulating Triiodothyronine Concentrations are Maintained despite Severe Hypothyroidism in Growing Pigs Fed Rapeseed Presscake Meal^1,2,3,

Division of Nutrition Pathology, Institute of Animal Breeding ^* Institute of Veterinary Pathology, School of Veterinary Medicine, University of Berne, 3012 Berne, Switzerland

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.

KEY WORDS: • rapeseed meal • pigs • thyroid • 5'-monodeiodinase • thyroid hormones

¹ Part of a thesis by C. Spiegel, accepted by the Veterinary Faculty of Berne, December 1991. Presented in part at the 43rd Annual Meeting of the "Gesellschaft für Haustierphysiologie," April 1991, Giessen, Germany [Spiegel, C., Bestetti, G., Rossi, G. & Blum, J. W. (1991) Fütterung von Rapspresskuchenmehl und perorale Substitution mit Thyroxin bei Mastschweinen: Effekte auf Wachstum, Schilddrüse und Leber sowie hepatische 5'-Monodejodaseaktivität. J. Anim. Physiol. Anim. Nutr. 66: 181 (abs.)].

² Supported by a grant (no. 012.89.6) from the Swiss Federal Veterinary Administration.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed.

Manuscript received 30 December 1992. Revision accepted 20 May 1993.