Here is a mystery. Before 1979 thyroid disease in cats was unknown. Since then cats have increasingly been diagnosed with overactive thyroids (hyperthyroidism). Today many middle aged cats and 10% of those over 10 years show symptoms. Curiously, while hyperthyroidism is uncommon in dogs, dogs do have a problem with underactive thyroids (hypothyroidism).
What is causing this epidemic of hyperthyroidism?
There is some good evidence of a link with dust from flame retardant chemicals (PBDE’s) used in soft furnishings, electronics and other household items. These chemicals turn out to be very similar to thyroid hormones and can interact with them. Dr Mark E Petersen — who was the first person to spot the existence of the condition in cats as well as pioneering their treatment — writes in Insights into Veterinary Endocrinology:
In both man and experimental animals, PBDEs clearly disrupt thyroid hormone metabolism… In dust from homes of hyperthyroid cats, total PBDEs concentrations were significantly higher
He also points out:
Another issue concerns our own heath. Shouldn’t we all be concerned about our own PBDE exposure? Remember that this chemical doesn’t act as a thyroid disruptor only in cats— it also affects all of us.
Now, however, another disturbing possibility has been raised which involves flavonoids. For humans the flavonoids are considered healthy as they are an antioxidant, though they also are believed to have some effect on the human thyroid. But researchers are questioning whether they could also be involved in the increase in feline thyroid conditions.
Since flavonoids come from plants this would not normally be relevant to cats as they would never naturally eat them. The problem arises only because many cat foods are now loaded with grain to bulk-up volume. The following update has just been given by the Morrison Foundation who fund animal research:
Researchers from the University of Georgia are examining whether cat food ingredients play a role in disease development. In the laboratory, the researchers treated feline thyroid cell cultures with various cat food ingredients to determine whether these ingredients stimulate normal thyroid cells. They learned that flavonoids—plant proteins found in commercially available cat food—activate cultured feline thyroid cells as effectively as a cat’s normal thyroid-stimulating hormone. This suggests that flavonoids may interfere with normal thyroid function and be a contributing factor in the development of feline hyperthyroidism.
The researchers are now confirming their work and the results should be available in July. Doubtless should the result confirm the present finding then cat food manufacturers will rush to remove flavonoids from their products. We hope so anyway, and will certainly do what we can to campaign for this should they not respond.
This does not, of course, solve the problem of the PBDE’s which still appear a major contributory factor. In the meantime all cat lovers should be aware of the problem.
According to Dr Petersen the symptoms of feline hyperthyroidism include “anxiety or nervousness, a rapid, pounding heart rate, muscle weakness, vomiting, diarrhea, increased thirst or urination, heat intolerance (panting), and an unkempt hair coat.” Cats may eat ravenously and still lose weight. Veterinarians can make referrals to his clinic: the link is Animal Endocrine Clinic and it contains a wealth of useful information.