One of the great mysteries of veterinary medicine if hyperthyroidism in cats. Benign tumors of the thyroid glad in older cats can produce excessive amount of thyroid hormone, and this can lead to a host of clinical symptoms. Fortunately, the disease is relatively easily treated by various methods. However, the condition seems to be more common than it used to be a few decades ago (though there are no reliable statistics, and it could just as easily be that vets were less likely to diagnose this problem in the past), so this begs the question of what might be the cause of the apparent increase in cases.
There are many theories, and environmental toxins of one kind of another are a popular focus of blame for this disease. In 2015 I wrote about the claim that certain flame retardant chemicals, known as PBDEs, might be associated with thyroid disease in cats, and I provided an update on the literature about this in 2017. At this point, the best we can say is that there has been some association between PBDE levels and thyroid disease in cats, but it is based on very weak data involving few animals, and no strong evidence yet exists to label this as a causal factor.
I recently ran across a new article by the same research group who looked at PBDEs and thyroid disease in 2016, this time suggesting there might be a connection with a different class of environmental chemicals, known as per- and polyfluoroalkyl substances (PFASs for short).
Miaomiao Wang, Weihong Guo, Steve Gardner, Myrto Petreas, June-Soo Park. Per- and polyfluoroalkyl substances in Northern California cats: Temporal comparison and a possible link to cat hyperthyroidism. Environmental Toxicology and Chemistry, 2018; DOI:
This paper appears to be using blood samples from the same groups of cats discussed in the previous paper, 26 sampled between 2008 and 2010, and 22 sampled between 2012 and 2013. The conclusions are similar but even more tentative than those regarding PBDEs. Levels of the chemicals declined between the earlier sampling period and the later (in humans as well as cats), likely due to the phasing out of these chemicals in industrial use.
There was a statistical associated between PFAS levels and thyroid disease in the latter period, but not in the earlier period. However, there was tremendous individual variation and very few cats in the two groups (those with thyroid disease and those without), so it is impossible to identify a clearly meaningful difference. Whether these chemicals even could be a potential cause for hyperthyroidism is unclear. While some lab animal studies have shown that PFASs can affect thyroid hormone levels, the direction of this effect (towards higher or lower levels) and the real-world significance are not at all clear in ab animals or humans.
Interestingly, there was no apparent association between levels of PFASs and PBDEs in these cats. That means that while PBDEs may have been associated with hyperthyroidism, and PFASs may have been associated with hyperthyroidism, there was no association between PBDEs and PFASs. That doesn’t make a lot of statistical sense, and it is likely an example of the limitations of such a small sample of cats.
While papers like these are interesting, they don’t yet add up to any clear or compelling evidence for a causal role of either of these classes of chemicals in feline hyperthyroidism. If anything, they may suggest such a role is unlikely since the levels of both PFASs and PBDEs seem to be going down over the last 10 years, yet there is no sign that thyroid disease is becoming any less common.
Has anyone with no political agenda been funded to look at this problem? If it’s not as we first thought not related to treatments but is environmental caused I think it should be studied from the human risk worry.
I have two cats, a male tabby and a female Siamese, and both have hyperthyroid. They are indoor/outdoor. I suspect an environmental issue, but I don’t use pesticides, or herbicides on my property, but neighbors probably do. It is highly suspect that both developed the disease.
As unfortunate as this is for you and your cats, it is not at all unlikely for two cats in the same household to develop this disease as it is extremely common in cats generally. If it were as simple as finding a single common exposure among cats with the disease, we would have figured it out long ago, but it is likely the cause is complex and involves multiple factors. Bad luck is the most likely answer for what has happened to your kitties.