Hyperthyroidism in cats: what's causing this epidemic of thyroid disease and can we prevent it?

Abstract

Practical relevance: Since first being reported in the late 1970s, there has been a dramatic increase in the prevalence of hyperthyroidism in cats. It is now recognized worldwide as the most common feline endocrine disorder.

Patient group: Hyperthyroidism is an important cause of morbidity in cats older than 10 years of age. It is estimated that over 10% of all senior cats will develop the disorder.

Clinical challenges: Despite its frequency, the underlying cause(s) of this common disease is/are not known, and no one has suggested a means to prevent the disorder. Because of the multiple risk factors that have been described for feline hyperthyroidism, it is likely that more than one factor is involved in its pathogenesis. Continuous, lifelong exposure to environmental thyroid disruptor chemicals or goitrogens in food or water, acting together in an additive or synergistic manner, may first lead to euthyroid goiter and then to autonomous adenomatous hyperplasia, thyroid adenoma and hyperthyroidism.

Evidence base: This review draws on published research studies to summarize the available evidence about the risk factors for feline hyperthyroidism. Based on the known goitrogens that may be present in the cat's food, drinking water or environment, it proposes measures that cat owners can implement that might prevent, or reduce the prevalence of, thyroid tumors and hyperthyroidism in their cats.

Figure 1

Over the past 30 years, the hyperthyroid cat has become a familiar sight in veterinary clinics around the world

Figure 2

Palpation of the thryoid to reveal a thyroid nodule in a cat with suspected hyperthyroidism

Figure 3:

Thyroid hormone regulation, action and metabolism

Figure 4:

Target sites for iodine deficiency and thyroid chemical disruption

Figure 5

Chemical structure of bisphenol A (BPA), thyroxine (T ₄ ) and triiodothyronine (T ₃ )

Figure 6

Chemical structure of PBDEs, PCBs and thyroxine (T ₄ ). The similarity of PBDEs and PCBs to thyroid hormones may underlie the chemicals’ toxicity. All of the compounds consist of two six-carbon rings decorated with halogens. Bromine attaches to the carbon rings of PBDEs, chlorine to those of PCBs, and iodine to those of thyroid hormone. In PBDEs, an atom of oxygen bridges the rings, whereas the rings of PCBs and thyroid hormones are linked by carbon–carbon bonds

Figure 7

With regard to the cat’s diet, it is not only the food itself, but also the means of storage and the type of food bowl used that need to be considered