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 *
Am J Vet Res 2004;65:586–591

 
Abstract

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.