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.


METHODS
18 healthy adult cats.


METHODS
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
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.
White
Heidi L
HL
Department of Clinical Sciences, School of Veterinary Medicine, Tufts University, North Grafton, MA 01536, USA.
Freeman
Lisa M
LM
Mahony
Orla
O
Graham
Peter A
PA
Hao
Qin
Q
Court
Michael H
MH
D000704:Analysis of Variance; D000818:Animals; D001772:Blood Cell Count; D001774:Blood Chemical Analysis; D002415:Cats / Q000097:blood / Q000502:physiology*; D018592:Cross-Over Studies; D007529:Isoflavones / Q000652:urine; D045730:Soy Foods; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*
D007529:Isoflavones; D014284:Triiodothyronine; D013974:Thyroxine