Thyroid Tumors Grow Progressively Larger in Most Hyperthyroid Cats Treated with Methimazole

As I discussed in my last post (Managing Hyperthyroid Cats That Become Unresponsive to Methimazole), methimazole blocks thyroid hormone secretion from a hyperthyroid cat's thyroid tumor (usually a benign tumor). However, in cats treated with methimazole, it is quite common for hyperthyroid cats on methimazole treatment to need higher dosages of methimazole over time, as the thyroid adenoma continues to grow larger and increases its secretion of thyroid hormone (1-3).

No one has yet studied the long-term effects of nutritional management with a low-iodine diet (i.e., Hill's y/d) on thyroid tumor growth in these hyperthyroid cats. However, the same progressive growth of the thyroid tumor would be expected on a low-iodine diet, since the thyroid tumor remains and is free to continue to grow progressively larger with time.

Figure 1: Hyperthyroid cat who has developed a very large thyroid tumor after 3 years of methimazole treatment. Notice the swollen neck, which turned out to be a massive thyroid carcinoma.

After months to years of methimazole treatment, many of these cats will develop a very large, palpable thyroid tumor (Figure 1) and will become difficult to regulate, even with high daily doses of oral or transdermal methimazole (3-5). Some cats eventually become completely refractory to methimazole, so alternative treatment modalities must be considered.

With enough time and as the disease progresses, the benign thyroid adenoma characteristic of early feline hyperthyroidism can also transform into malignant thyroid carcinoma in some cats (5,6). Again, methimazole or other antithyroid drug therapy (including Hill's y/d) does nothing to the thyroid tumor pathology and cannot stop this from happening.

Radioiodine therapy can be used to successfully treat cats with all sizes of benign and malignant thyroid tumors, but is best used on cats with small benign tumors. Cats with small thyroid tumors are easier to cure with lower radioiodine doses, resulting in shorter hospitalization times.

Video Animation: Thyroid Growth on Methimazole

The animation below describes the continued growth of the initially benign thyroid tumors causing hyperthyroidism in cats during their medical management with methimazole (Tapazole).

This video was done by my colleague and friend, Dr. Michael Broome, Director of Advanced Veterinary Medical Imaging in Orange Country, California. We have been long-term collaborators on clinical research issues pertaining to cats with hyperthyroidism, and we continue to work closely together on this common disorder (4,5).




References:

1. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110. 
2. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012;571-592. 
3. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
4. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2012; 26:754.
5. Peterson ME, Broome MR. Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. Proceedings of European College of Veterinary Internal Medicine; 2012.
6. 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.