Relationship Between Stomatitis and Hyperthyroidism in Cats?

My patient is a 12-year-old, male hyperthyroid cat who had a full mouth extraction done 4 years ago because of severe oral disease. Since that time, this cat had required Depo-medrol injections every 1-2 months to help keep the stomatitis under control.

About a year ago, the cat developed hyperthyroidism and was treated with radioiodine. Since that time, he has remained euthyroid. But unexpectedly, his stomatitis has also completely resolved, and no further steroid treatment has been needed.

Nothing else changed in the household — i.e., no diet changes, no other medications, and this has always been an indoor cat. I am happy for the cat (and the owner), but am curious if you have an explanation to explain the apparent connection between the stomatitis and hyperthyroidism?

My Response

I've seen a few hyperthyroid cats over the years whose concurrent oral disease improved after they were treated with radioiodine. I do not know for certain why their oral lesions sometime respond so well to correction of the hyperthyroid state.

Are some hyperthyroid cats iodine deficient?
One theory is that some of these hyperthyroid cats are iodine deficient (even on a "normal" iodine diet) because of their increased need for iodine to make thyroid hormone (1). Once we cure them with radioiodine, their need for iodine goes down because now they are no longer making excess amount of thyroid hormone.

Figure 1. Chemical structure of thyroxine.
Notice that each T4 molecule contains 4 atoms of iodine atoms 

Remember that each T4 molecule contains 4 iodine atoms (Figure 1), whereas each T3 molecule contains 3 iodine atoms.

Salivary glands normally concentrate iodine
Iodine appears to be very important for oral health (2,3). Normally, the salivary glands have the capacity to concentrate iodide as well as the thyroid tissue. The iodide is then secreted into saliva such that its salivary concentration has been reported to vary from 20 to 100 times that found in the serum (4,5).

Figure 2. Thyroid scintigraphy of a normal cat.
Notice the similar uptake of the tracer in the thyroid lobes and salivary glands. Also note that the stomach also concentrates the radionuclide to some extent (area of uptake on bottom of scan image).

In proof of this, remember that the salivary glands can be seen to concentrate ("take up") radioiodine with thyroid scintigraphy very well (see Figure 2) —equal to the thyroid glands in a normal cat (6-8).

Iodine deficiency and immunity
Based on a number of studies in man and other species, it's clear that iodine has many extra-thyroidal functions in the body. Iodine plays an important role in the immune response. It also has anti-inflammatory and anti-oxidative actions (2,3,9-12).

Bottom Line

Marked variation of iodine levels between cat foods have been reported in studies of commercial foods, in which the iodine content between foods varied by a factor of 30-fold (13-15). It also possible that some cats would be consistently fed a low-iodine diet for prolonged periods, leading to an iodine-deficient state.

So it is possible that this cat has been chronically iodine deficient, which may have become worse when hyperthyroidism developed. By treating the hyperthyroidism, the total body need for iodine would be expected to decrease. So that might explain why the stomatitis improved after correction of the hyperthyroidism with radioiodine.

Again, I'm not talking about the use of low-iodine diets (y/d) for hyperthyroidism here, but that diet could, at least theoretically, make the oral disease worse in these cats.

References:

1. Wakeling J, Elliott J, Petrie A, et al. Urinary iodide concentration in hyperthyroid cats. American Journal of Veterinary Research 2009;70:741-749. 
2. Venturi S, Venturi M. Iodine in evolution of salivary glands and in oral health. Nutrition and Health 2009;20:119-134. 
3. Brownstein, D. Iodine: Why you need it, Why you can't live without it. Medical Alternative Press; 3rd edition. 2004.
4. Myant NB. Iodine metabolism of salivary glands. Annals of the New York Academy of Sciences 1960;85:208-214.
5. Mason DK, Harden R McG, Alexander WD. The salivary and thyroid glands: A comparative study in man. British Dental Journal 1967;122:485-489. 
6. Beck KA, Hornof WJ, Feldman EC. The normal feline thyroid. Technetium pertechnetate imaging and determination of thyroid to salivary gland radioactivity ratios in 10 normal cats. Veterinary Radiology & Ultrasound 1985;26:35-38. 
7. Peterson ME, Becker DV. Radionuclide thyroid imaging in 135 cats with hyperthyroidism. Veterinary Radiology 1984;25:23-27.
8. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:10-16.
9. Marani L, Venturi S, Masala R. Role of iodine in delayed immune response. Israeli Journal of Medical Science 1985;21:864. 
10. Venturi S, Venturi M. Iodine, thymus, and immunity. Nutrition 2009;25:977-979.
11. Miller DW. Extrathyroidal benefits of iodine. Journal of American Physicians and Surgeons 2006;11:106-110. 
12. Ware CM, Wishner LA. The lipid antioxidant properties of iodine compounds. Lipids 1968;3:182-183.
13. Johnson LA, Ford HC, Tarttelin MF, et al. Iodine content of commercially-prepared cat foods. New Zealand Veterinary Journal 1992;40:18-20. 
14. Ranz D, Tetrick M, Opitz B, et al. Estimation of iodine status of cats. Journal of Nutrition 2002;132 (suppl 2):1751S-1753S. 
15. 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.