New study may shed light on cause of uptick of hyperthyroidism in cats

A few months ago, a client updated her cat's medical information for my files, as he had been recently diagnosed with hyperthyroidism. Two of her three cats now have the condition. 

"Have you ever heard any theories on hyperthyroidism in cats, in particular when there is a herd diagnosis?" she said.

"The veterinarian mentioned to me that there has been some research on this, but they can't determine the cause."

I found this curious myself, because I've seen an uptick in cases with my charges over the past few years (typically older cats) and I know that the overall rate amongst felines is increasing. 

The condition occurs when there's an increase in production of thyroid hormones from thyroid glands, which are located in the neck. Characterized subtly at first, symptoms can include weight loss, increased appetite, increased thirst and resulting frequent urination, among other tell-tale signs. Treatment is crucial (luckily, transdermal meds are an option) as the excess production of the thyroid hormones known as T3 and T4 can affect heart function as well as other organs, like the kidneys.

Some studies have hinted that there is a connection between hyperthyroidism in cats and flame retardants – synthetic chemical compounds, like polybrominated diphenyl esters (PBDEs) – and subsequently the products that they are used on.  

Researchers in Japan, based on a study published in Environmental Science and Technology suggest that those synthetic chemical sources may not be as big an influence on the incidence of feline hyperthyroidism as first thought. 

An unlikely source – cat food, but more specifically, those varieties containing fish – may be the culprit. As it turns out, marine organisms contain naturally-occurring compounds similar in composition to synthetic PBDEs. 

The team, led by Hazuki Mizukawa of Ehime University, tested not just the blood samples of cats, but cat food. Using liver microsomes, they were able to simulate how the feline body might metabolize the compounds. They were also able to isolate the concentrations of specific metabolites, like MeO-PBDEs.

Quoting the study: 

The present study suggests that pet cats are exposed to MeO-PBDEs through cat food products containing fish flavors and that the OH-PBDEs in cat blood are derived from the CYP-dependent demethylation of naturally occurring MeO-PBDE congeners, not from the hydroxylation of PBDEs. 

The team's findings indicate that the high levels of the naturally-occurring compounds found in both the fish-flavored food and the blood samples of the cats could in some way shed more light on the increase in feline hyperthyroidism, though more research is needed. 

Click here to read the study titled, Organohalogen Compounds in Pet Dog and Cat: Do Pets Biotransform Natural Brominated Products in Food to Harmful Hydroxlated Substances?

Lorrie Shaw is a freelance writer and owner of Professional Pet Sitting. Shoot her an email, contact her at 734-904-7279 or follow her adventures on Twitter.