Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats

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 (T-4) and free T-4 (fT(4)) concentrations, but unchanged total triiodothyronine (T-3) concentrations. The T-3/fT(4) ratio was also significantly lower in cats that received the soy diet. Although the magnitudes of the increases were small (8% for T-4 and 14% for fT(4)), these changes resulted in an increased proportion of cats (from 1/18 to 4/18) that had fT(4) 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 T-4 concentration relative to T-3 concentration may result from inhibition of 5'-iodothyronine deiodinase or enhanced T-3 clearance. Soy is a common dietary component that increases serum T-4 concentration in cats.