Abstract
American Journal of Veterinary Research
May 2004, Vol. 65, No. 5, Pages 586-591
doi: 10.2460/ajvr.2004.65.586
Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats
Heidi L. White , DVM Lisa M. Freeman , DVM, PhD Orla Mahony , DVM Peter A. Graham, , BVMS, PhD Qin Hao , MS Michael H. Court , BVSc, PhD
Department of Clinical Sciences, School of Veterinary Medicine, Tufts University, North Grafton, MA 01536. (White, Freeman, Mahony); Veterinary Diagnostic Endocrinology, Diagnostic Center for Population and Animal Health (AHDL), Michigan State University, Lansing, MI 48909. (Graham,); Present address is NationWide Laboratories, 23 Mains Lane, Poulton-Le-Fylde, Lancashire FY6 7LJ, UK. (Graham,); Department of Pharmacology and Experimental Therapeutics, School of Medicine, Tufts University, Boston, MA 02111. (Hao, Court)
Objective—To compare effects of short-term administration of a soy diet with those of a soy-free diet on serum thyroid hormone concentrations in healthy adult cats.
Animals—18 healthy adult cats.
Procedure—Cats were randomly assigned to receive either a soy or soy-free diet for 3 months each in a crossover design. Assays included CBC, serum biochemical profile, thyroid hormone analysis, and measurement of urinary isoflavone concentrations.
Results—Genistein, a major soy isoflavone, was identified in the urine of 10 of 18 cats prior to dietary intervention. Compared with the soy-free diet, cats that received the soy diet had significantly higher total thyroxine (T4) and free T4 (fT4) concentrations, but unchanged total triiodothyronine (T3) concentrations. The T3/fT4 ratio was also significantly lower in cats that received the soy diet. Although the magnitudes of the increases were small (8% for T4 and 14% for fT4), these changes resulted in an increased proportion of cats (from 1/18 to 4/18) that had fT4 values greater than the upper limit of the laboratory reference range. There was no significant effect of diet on any other measured parameter.
Conclusions and Clinical Relevance—Short-term administration of dietary soy has a measurable although modest effect on thyroid hormone homeostasis in cats. Increase in T4 concentration relative to T3 concentration may result from inhibition of 5'-iodothyronine deiodinase or enhanced T3 clearance. Soy is a common dietary component that increases serum T4 concentration in cats. ( Am J Vet Res 2004; 65:586–591)
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