Hyperthyroidism is the most common hormonal disease in cats in the United States. The cause of the disease is unknown, but the incidence of the disease has dramatically increased in the last 20 years. This suggests there is some factor that is responsible for predisposing cats to development of this disease. Suggested factors include:
- Infectious organisms
- Environmental chemicals
- Changes in diet
It has been suggested that newer cat foods may contain components called goitrogens, which increase the activity of the thyroid gland and could be responsible for the development of feline hyperthyroidism. Goitrogens may include iodine and soy.
Researchers decided to compare the effects of short-term administration of a soy diet with those of a soy-free diet on serum thyroid hormone concentrations in 18 healthy adult cats. The cats were randomly assigned to receive either a soy or soy-free diet for 3 months each. Blood tests were periodically performed to monitor thyroid hormone levels. At the end of the 3 months, each cat was switched to the other diet, and again monitored for 3 months.
The short-term administration of dietary soy had a measurable although modest effect on thyroid hormone levels in cats. The researchers concluded that soy is a common dietary component that increases a certain thyroid hormone called "T4" concentration in cats. The clinical implication of this effect, particularly with regard to the development of feline hyperthyroidism, is presently unclear and will require further study before a definitive conclusion can be reached.