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
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;