Earlier studies have shown that phytoestrogens are found in cat foods containing soybean and soy isoflavones. This study was done to compare the effects of short-term consumption of soy diets with consumption of soy-free diets on serum thyroid hormone concentrations in healthy adult cats. This was a crossover study in which the cats received a soy diet for 3 months and then a soy-free diet for another 3 months. Thyroid hormone analyses, CBCs, serum biochemistry analysis, and measurement of urinary isoflavones were done. The soy diet had a modest effect on the serum thyroid concentrations. These cats were fed the diets for only 3 months, yet some had free thyroxine values greater than the upper limit of the laboratory reference range. This phenomenon needs further study to determine the long-term effects of soy on the feline thyroid gland.

COMMENTARY: Hyperthyroidism is the most common endocrine disease of cats in the United States. There have been many studies and theories on the dramatic increase in cases that occurred in the 1980s and 1990s. This study suggests that soy may be one of the factors involved.

Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats. White HL, Freeman LM, Mahony O, et al. AM J VET RES 66:586-591, 2004.