title: Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats.
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.
  
  
  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.
authors.affiliation: Department of Clinical Sciences, School of Veterinary Medicine, Tufts University, North Grafton, MA 01536, USA.
authors.forename: Heidi L
authors.initials: HL
authors.lastname: White
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Freeman
authors.forename: Orla
authors.initials: O
authors.lastname: Mahony
authors.forename: Peter A
authors.initials: PA
authors.lastname: Graham
authors.forename: Qin
authors.initials: Q
authors.lastname: Hao
authors.forename: Michael H
authors.initials: MH
authors.lastname: Court
mesh_terms: 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*
chemical_list: D007529:Isoflavones; D014284:Triiodothyronine; D013974:Thyroxine