Treating Hyperthyroid Cats with Radioiodine: The Pros and Cons

Hyperthyroidism is the most common endocrine disorder in cats, most frequently associated with adenomatous hyperplasia (or adenoma) involving one or both thyroid lobes (1,2). Because the exact pathogenesis of hyperthyroidism in cats is not known, treatment of the condition is directed at controlling the excessive secretion of thyroid hormone from the adenomatous thyroid gland.

Treatment options for cats with hyperthyroidism
Four treatment options are available for cats with hyperthyroidism (3-8):

• Long-term antithyroid drug administration
• Chronic feeding of an iodine-deficient diet (Hill’s y/d)
• Surgical thyroidectomy
• Administration of radioiodine (131-I) to irradiate and destroy the hyperfunctional thyroid nodule(s)

Each of these treatment options has its advantages and disadvantages, but the use of radioiodine is considered by most authorities to be the treatment of choice for the majority of hyperthyroid cats.

The best treatment option for a hyperthyroid patient is determined by evaluation of age, concurrent medical problems (such as cardiovascular or renal disease), availability of therapy, and the owner’s opinion and financial options (3,5,6,9).

Advantages of radioactive iodine (I-131) as treatment of cats with hyperthyroidism
Radioactive iodine (radioiodine; I-131) provides a simple, effective, and safe treatment for cats with hyperthyroidism.  This form of therapy has many advantages over other treatment methods (4-9).

• Radioiodine avoids inconvenience of daily, oral administration of an antithyroid drug as well as the side effects commonly associated with these drugs. 
• Radioiodine avoids the restrictions associated with the lifelong feeding of an iodine-deficient diet. 
• Radioiodine also eliminates the risks and perioperative complications associated with anesthesia and surgical thyroidectomy.
• A single administration of radioiodine restores euthyroidism in most (>90%) hyperthyroid cats. 
• The therapy is simple and relatively stress-free for most cats. 

Disadvantages of radioactive iodine (I-131) as treatment of cats with hyperthyroidism
Although the therapy is simple and relatively stress-free for cats, there are also a few downsides of radioiodine treatment for some cats.

• The use and treatment with radioiodine requires special radioactive licensing and facilities, nuclear medicine equipment, and extensive compliance with local and state radiation safety laws.
• Major drawback for most owners is that their cat must be kept hospitalized for a period (3 to 10 days in most treatment centers) and visiting is not allowed.
• The cats must be stable enough to undergo this procedure. If severe cardiac or renal disease is present, cats may not do well during this hospitalization period.
• If cats are underdosed with radioiodine, they will remain persistently hyperthyroid and will requrie additional treatment.
• If the cats are overdosed with radioiodine, on the other hand, hypothyroidism may develop (8,10).

Bottom Line

Overall, the use of radioactive iodine provides a simple, effective, and safe treatment for cats with hyperthyroidism. Unlike methimazole or nutritional therapy (iodine deficient diet), use of radioiodine cures the disease. It is regarded by most veterinarians to be the treatment of choice for most cats with hyperthyroidism.

Radioiodine is a particularly useful treatment for cats with bilateral thyroid involvement (found in approximately 70% of cats), cats with intrathoracic (e.g., ectopic) thyroid tissue, cats that fail to respond adequately to medical or nutritional management, and the relatively rare feline patient with thyroid carcinoma (8,11,12).

There are different protocols that facilities may use to determine the cats’ radioiodine dosage, which greatly influences the prevalence of persistent hyperthyroidism (when the administered 131-I dose is too low) and iatrogenic hypothyroidism (when the administered 131-I dose is too high). The method of dose calculation should be considered when selecting a radioiodine facility.

References

1. Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Vet Clin North Am Small Anim Pract 1994;24:541-565. 
2. Peterson M. Hyperthyroidism in cats: What's causing this epidemic of thyroid disease and can we prevent it? J Feline Med Surg 2012;14:804-818. 
3. Panciera DL, Peterson ME, Birchard, SJ: Diseases of the thyroid gland. In: Birchard SJ, Sherding RG (eds): Manual of Small Animal Practice (Third Edition), Philadelphia, Saunders Elsevier, pp 327-342, 2006.  
4. Peterson ME. Radioiodine treatment of hyperthyroidism. Clin Tech Small Anim Pract 2006;21:34-39.
5. Mooney CT, Peterson ME. Feline hyperthyroidism In: Mooney CT, Peterson ME, eds. Manual of Canine and Feline Endocrinology, Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;199-203.
6. Baral R, Peterson ME: Thyroid gland disorders, In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders, 2012;571-592.
7. Peterson ME. Hyperthyroidism in cats In: Rand JS, Behrend E, Gunn-Moore D, et al., eds. Clinical Endocrinology of Companion Animals. Ames, Iowa Wiley-Blackwell, 2013;295-310.
8. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism. In: Bonagura JD,Twedt DC, eds. Current Veterinary Therapy XIIII. Philadelphia: Saunders Elsevier, 2013: in press.
9. Kintzer PP. Considerations in the treatment of feline hyperthyroidism. Vet Clin North Am Small Anim Pract 1994;4:577–585.
10. Nykamp SG, Dykes NL, Zarfoss MK, et al. Association of the risk of development of hypothyroidism after iodine 131 treatment with the pretreatment pattern of sodium pertechnetate Tc 99m uptake in the thyroid gland in cats with hyperthyroidism: 165 cases (1990-2002). J Am Vet Med Assoc 2005;226:1671-1675.
11. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Feline Med Surg 2009;11:116-124. 
12. Turrel JM, Feldman EC, Nelson RW, et al. Thyroid carcinoma causing hyperthyroidism in cats: 14 cases (1981-1986). J Am Vet Med Assoc 1988;193:359-364.