Effects of dietary soy isoflavones on health, steroidogenesis, and thyroid gland function in dogs

Objective-To evaluate the effect of a soy-based diet on general health and adrenocortical and thyroid gland function in dogs. Animals-20 healthy privately owned adult dogs. Procedures-In a randomized controlled clinical trial, dogs were fed a soy-based diet with high (HID; n = 10) or low (LID; 10) isoflavones content. General health of dogs, clinicopathologic variables, and serum concentrations of adrenal gland and thyroid gland hormones were assessed before treatment was initiated and up to 1 year later. Differences between groups with respect to changes in the values of variables after treatment were assessed by means of a Student t test (2 time points) and repeated-measures ANOVA (3 time points). Results-No differences were detected between the 2 groups with respect to body condition and results of hematologic, serum biochemical, and urine analyses. Most serum concentrations of hormones did not change significantly after treatment, nor were they affected by diet. However, the mean change in serum concentration of total thyroxine was higher in the HID group (15.7 pmol/L) than that in the LID group (-1.9 pmol/L). The mean change in estradiol concentration after ACTH stimulation at I year after diets began was also higher in the HID group (19.0 pg/mL) than that in the LID group (-5.6 pg/mL). Conclusions and Clinical Relevance-Phytoestrogens may influence endocrine function in dogs. Feeding soy to dogs on a long-term basis may influence results of studies in which endocrine function is evaluated, although larger studies are needed to confirm this supposition. (Am J Vet Res 2009;70:353-360)