Success Rates for Radioiodine Treatment in Hyperthyroid Cats

The ideal goal of radioiodine (131-I) therapy in cats with hyperthyroidism is to restore euthyroidism with a single dose of radioiodine without producing hypothyroidism. Indeed, most hyperthyroid cats treated with 131-I are cured by a single dose (1-7).

Recommended monitoring after radioiodine
In general, the cats should be monitored at 2 to 4 weeks and again at 3 months after discharge from the radioiodine facility (4,7). At both of these recheck times, a complete physical examination as well as routine laboratory testing (e.g., CBC, serum chemistry panel, urinalysis) and serum thyroid hormone determinations (i.e, a total T4 concentration at minimum) are recommended.

If the serum T4 falls to subnormal or low-normal values (< 1.5 µg/dl) and iatrogenic hypothyroidism is suspected, a complete thyroid profile (total and free T4, T3, cTSH concentrations) is recommended to help rule out that diagnosis (4,8). I'll be discussing transient and permanent hypothyroidism more in my upcoming posts.

Cure rate after radioiodine
Serum thyroid hormone concentrations are normal within 2 to 4 weeks of radioiodine treatment in approximately 85% of cats and in 95% of cats by 3 months (5,6). Although cats appear to feel better within days after treatment, the owner should notice gradual clinical improvement and resolution of the signs of hyperthyroidism during this 3-month period.

Note that these percentages indicate remission of the hyperthyroid state but do not reflect the incidence of iatrogenic hypothyroidism, which has a reported incidence as high as 30% in some studies (9). Other studies, which employ serum TSH values to help identify early hypothyroidism, indicate that the rate of hypothyroidism can be much higher (up to 80%), depending on the method of dose determination. (10).

Persistent hyperthyroidism—Causes of treatment failure 
Approximately 5% of cats fail to respond completely and remain hyperthyroid after treatment with radioiodine. In studies from my clinic, most cats with persistent hyperthyroidism have large thyroid tumors, severe hyperthyroidism, and very high serum T4 concentrations (11). Other cats with mild-to-moderate hyperthyroidism may have a lower-than-expected thyroid 131-I uptake or show rapid turnover of the administered 131-I by the thyroid tumor (see below). In all these instances, treatment failure results because the radiation dose delivered to the tumor was inadequate to completely ablate the adenoma.

During the first 3 to 5 days of treatment with 131-I, we routinely measure daily neck radiation levels as an approximation of the cat’s thyroid iodine uptake value. This is useful in estimating the 131-I residence time in the thyroid tumor and can help determine the cause of treatment failure. In most hyperthyroid cats, the maximal thyroid radiation level is reached between 24 and 48 hours. 

Occasionally, a hyperthyroid cat shows an early peak thyroid uptake reading (i.e., sooner than 24 hours after dose administration), with lower thyroid uptake values at 24 and 48 hours. This early peak thyroid radioiodine uptake with rapid clearance is defined as “rapid iodine turnover” by the thyroid tumor (12). Such rapid turnover implies a short residence time for 131-I in the thyroid gland, which indicates that the administered 131-I dose may have a diminished radiation effect on the adenomas. In these cats, increased therapeutic 131-I dosages are needed to compensate for the decreased radiation effect to reduce the risk of persistent disease.

Persistent hyperthyroidism—Retreatment plan
If the hyperthyroid state persists in any cat for longer than 3 months after initial treatment, retreatment with radioiodine should be considered. In such cats, thyroid imaging (thyroid scintigraphy) and determination of thyroid uptake may help determine the cause of the initial treatment failure and ensure success with the second 131-I treatment.

In most of these cats, a 131-I dose (higher than that which was originally administered) will be needed to cure their hyperthyroid state. The prognosis remains good, however, and almost all can be cured with the second dose of radioiodine.

References:

1. Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
2. 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.
3. 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.
4. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD,Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013;in press.
5. Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. J Am Vet Med Assoc 1995;207:1422-1428.  
6. Slater MR, Komkov A, Robinson LE, et al: Long-term follow up of hyperthyroid cats treated with iodine-131. Vet Radiol Ultrasound 1994;35:204-209.
7. Peterson ME. Radioiodine treatment of hyperthyroidism. Clin Tech Small Anim Pract 2006;21:34-39. 
8. Peterson ME. Diagnostic testing for feline thyroid disease: Hypothyroidism. Compend Contin Educ Vet 2013:in press.
9. 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.
10. Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. J Vet Intern Med 2010;24:1086-1092.
11. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats, in Conference Proceedings 29th Annual Veterinary Medical Forum (American College of Veterinary Internal Medicine) 2011;104-106.
12. Aktay R, Rezai K, Seabold JE, et al. Four- to twenty-four-hour uptake ratio: an index of rapid iodine-131 turnover in hyperthyroidism. J Nucl Med 1996; 37:1815-1819.

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