Hyperthyroidism is a very common disorder of older cats. Also called thyrotoxicosis, hyperthyroidism is caused by an increase in production of the two main thyroid hormones — known as thyroxine (T4) and triiodothyronine (T3) from an enlarged thyroid tumor.

Thyroid hormones affect nearly all the organs in the body; they play an important role in controlling the body's metabolic rate and thus the general activity level. This increased metabolic rate explains why hyperthyroid cats tend to burn up energy too rapidly and suffer weight loss despite having an increased appetite and food intake.

I first described the first cats with hyperthyroidism in 1979, only just over 30 years ago (1). Since then, hyperthyroidism has become the most common endocrine disorder in cats and a disease frequently diagnosed by all veterinarians in small animal practice. It is unclear why such a phenomenon has occurred.

Undoubtedly, increased awareness of hyperthyroidism by veterinary practitioners and by cat owners, easier availability of diagnostic tests, and a growing pet cat population have all played a role. However, its increased prevalence is not likely to be the result of an aging cat population alone. It may truly be a new disease that has just developed over the last three decades.

Incidence of feline hyperthyroidism varies around the world
The prevalence of hyperthyroidism varies geographically and at least anecdotally, regionally. It is a disease that is frequently diagnosed throughout North America, Europe, the United Kingdom, Australia, and New Zealand.

More recently, investigators in Japan have also reported an increasing number of cases. However, the disease appears to be much less common in other countries that have excellent veterinary care, such as in Spain and Hong Kong (2,3).

Thyroid gland changes associated with hyperthyroidism in cats
One of the largest endocrine gland in the body, the thyroid gland is found in the neck area, situated below the larynx (Adam's Apple) and adjacent to the trachea (windpipe).

In all species, including the cat, the thyroid gland is composed on 2 cone-like lobes or wings (in other words, there are 2 thyroid lobes that comprise the single thyroid gland). In most cats with hyperthyroidism, a benign or non-cancerous tumor (adenoma) develops in one or both of the thyroid lobes (4,5).

By contrast, thyroid cancer (carcinoma) is a rare cause of hyperthyroidism in cats, accounting for the tumors in less than 5% of hyperthyroid cats with hyperthyroidism (5,6).

But what’s causing these thyroid tumors to develop in the first place?
To date, the underlying factor(s) responsible for the thyroid changes remains obscure and is probably multifactorial. However, studies have indicated numerous environmental and nutritional risk factors, which may play a role in the why and how this disorder develops.

Possible risk factors:

- Diet composed entirely or primarily of canned cat food (7,9,15,18).
- Certain varieties of canned cat good, such as fish, liver, or giblet flavor (7, 10,18).
- Use of cat litter (7.9,18)
- Diets containing either excess or deficient amounts of iodine have been implicated (8, 17,19)
- Diets containing high amounts of selenium have also been suggested to have a role (11).
- Cans with plastic linings and pop-top lids may pose a greater risk than sachets or cans, which require a can opener to open them (13,18). This is potentially due to the release of chemicals such as bisphenol-A and bisphenol-F from the lacquer linings of the pop-top cans.
- Soy isoflavones (genistein and daidzein) are common constituents of commercially available cat foods and also may interfere with normal thyroid function (12,14).
- Regular use of insecticidal products (flea products) on the cat or fly sprays within the household (7,9,15).
- Exposure to herbicides and fertilizers (7,15).
- Exposure to flame-retardant chemicals contaminants including polybrominated diphenyl ethers (PBDEs). Excessive PBDEs have been identified in household dust from contaminated carpet padding, polyurethane foams, furniture and mattresses. High levels of PBDEs have both identified in the serum of both dogs and cats, indicating that this in a some environment contaminant (16,20).

So what’s the bottom line?
Given the variety of abnormalities and associations described it is likely that hyperthyroidism is a multifactorial disease. In other words, more than one factor is contributing to its development and pathogenesis.

It is also important to realize, however, that many of the risk factors listed above also exist in areas of the world where hyperthyroidism is considered relatively uncommon. This emphasizes the complexity of the problem in identifying the cause(s) of hyperthyroidism in cats.

References:

- Peterson ME, Johnson JG, Andrews LK. Spontaneous hyperthyroidism in the cat. Proceedings of the American College of Veterinary Internal Medicine 1979:p. 108.
- Wakeling J, Melian C, A. F, et al. Evidence for differing incidences of feline hyperthyroidism in London, UK and Spain. Proceedings of the 15th ECVIM-CA Congress 2005;2005.
- De Wet CS, Mooney CT, Thompson PN, et al. Prevalence of and risk factors for feline hyperthyroidism in Hong Kong. Journal of Feline Medicine and Surgery 2009;11:315-321.
- Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Veterinary Clinics of North America Small Animal Practice 1994;24:541-565.
- Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645, v.
- Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.
- Scarlett JM. Feline hyperthyroidism: A descriptive and case-control study. Preventive Veterinary Medicine 1988;6.
- Tarttelin MF, Ford HC. Dietary iodine level and thyroid function in the cat. Journal of Nutrition 1994;124:2577S-2578S.
- Kass PH, Peterson ME, Levy J, et al. Evaluation of environmental, nutritional, and host factors in cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1999;13:323-329.
- Martin KM, Rossing MA, Ryland LM, et al. Evaluation of dietary and environmental risk factors for hyperthyroidism in cats. Journal of the American Veterinary Medical Association 2000;217:853-856.
- Foster DJ, Thoday KL, Arthur JR, et al. Selenium status of cats in four regions of the world and comparison with reported incidence of hyperthyroidism in cats in those regions. American Journal of Veterinary Research 2001;62:934-937.
- Court MH, Freeman LM. Identification and concentration of soy isoflavones in commercial cat foods. American Journal of Veterinary Research 2002;63:181-185.
- Edinboro CH, Scott-Moncrieff JC, Janovitz E, et al. Epidemiologic study of relationships between consumption of commercial canned food and risk of hyperthyroidism in cats. Journal of the American Veterinary Medical Association 2004;224:879-886.
- White HL, Freeman LM, Mahony O, et al. Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats. American Journal of Veterinary Research 2004;65:586-591.
- Olczak J, Jones BR, Pfeiffer DU, et al. Multivariate analysis of risk factors for feline hyperthyroidism in New Zealand. New Zealand Veterinary Journal 2005;53:53-58.
- Dye JA, Venier M, Zhu L, et al. Elevated PBDE levels in pet cats: sentinels for humans? Environmental Science and Technology 2007;41:6350-6356.
- Wakeling J, Elliott J, Petrie A, et al. Urinary iodide concentration in hyperthyroid cats. American Journal of Veterinary Research 2009;70:741-749.
- Wakeling J, Everard A, Brodbelt D, et al. Risk factors for feline hyperthyroidism in the UK. Journal of Small Animal Practice 2009;50:406-414.
- Edinboro CH, Scott-Moncrieff JC, Glickman LT. Feline hyperthyroidism: potential relationship with iodine supplement requirements of commercial cat foods. Journal of Feline Medicine Surgery 2010;12:672-679.
- Venier M, Hites RA. Flame retardants in the serum of pet dogs and in their food. Environmental Science and Technology 2011.


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