Radioiodine Treatment in Cats: Patient Selection and Preparation

Routine diagnostic testing should always be performed by the primary-care veterinarian before referral for radioiodine treatment to determine if a cat is an appropriate candidate for this treatment (1-4). This is very important, inasmuch as these cats tend to be middle-aged to older and therefore may have other geriatric problems unrelated to the cat's hyperthyroidism.

Cats should be relatively stable before being considered for radioiodine therapy. Those that have clinically significant or unstable cardiovascular, renal, gastrointestinal, endocrine (e.g., diabetes), or neurologic disease may not be very good candidates for this treatment, especially because of the length of boarding required after the 131-I dose is administered to the cats.

Working up the hyperthyroid cat—Recommended testing for diagnosis and staging of the disease

In any cat with suspected hyperthyroidism, we have 2 goals in diagnostic testing (1-4):

1. Our primary goal is to make a definitive diagnosis of hyperthyroidism, This may not be as easy as cat owners or veterinarians may believe. None of the current thyroid function tests are prefect, and false-positive test results are not uncommon.
2. Our second goal is to exclude other problems (kidney disease, gastrointestinal disease, diabetes), all common in the middle-aged to older cat. In some hyperthyroid cats, these concurrent problems are more important than the hyperthyroidism itself and must be addressed immediately.

To acheive these two goals, the recommended pretreatment workup for hyperthyroid cats includes the following tests and procedures (5):

• Routine database, including a complete blood count (CBC), serum chemistry panel, and complete urinalysis.
• Pretreatment or untreated serum total T4 concentration (with the cat not on antithyroid drug treatment or a low-iodine diet).
• If the cat has been treated medically or nutritionally for longer than 1 to 2 months, the antithyroid medication or low-iodine diet may have to be discontinued for 5 to 7 days and another serum total T4 measured to determine the true severity of the cat's hyperthyroidism. 
• If hyperthyroidism is mild or a thyroid nodule cannot be palpated, a complete thyroid panel is recommended to help confirm hyperthyroidism. This includes determination of serum concentrations of total T4, free T4, total T3, and thyroid-stimulating hormone (TSH).  Untreated hyperthyroid cats generally have high total T4, free T4, or T3 levels, whereas serum TSH is almost always suppressed to undetectable concentrations (6-8).
• Chest radiography or cardiac ultrasonography (or both) should be performed if the cat has evidence of any clinically significant cardiac disease (especially pronounced heart murmur, arrhythmia, dyspnea, or jugular venous distention).
• If severe gastrointestinal signs are present (e.g., poor appetite, severe vomiting or diarrhea), an abdominal ultrasound should be done to help rule out other problems not related to hyperthyroidism.

Hyperthyroidism and kidney disease

If concurrent renal disease is suspected or known to be present, many recommend evaluating medical management before a more definitive means of treatment such as radioiodine (1-3,9). In these cats, a low starting dose (i.e., 1.25 mg orally once daily or divided twice daily) of methimazole with gradual dosage escalation is prudent, with monitoring (e.g., biochemical profile and total serum T4 determination) and dose adjustments done every 2 weeks.

Once euthyroidism has been maintained for 2 to 4 weeks, no further fall in glomerular filtration rate (GFR) or acute worsening in renal function is expected, allowing one to decide whether to proceed with definitive therapy. Even if early or mild chronic kidney disease (CKD) is uncovered during this methimazole trial, most cats remain good candidates for radioiodine treatment (10).  Remember that hyperthyroidism contributes to the development of the renal disease seen in hyperthyroid cats so control of the hyperthyroidism may help slow the progression of the concurrent kidney disease (11).

Stabilizing the cat for radioiodine treatment

The veterinarian may choose to stabilize some cats for a few weeks or months before the time of referral for radioiodine treatment by administering β-blocking agents (e.g., atenolol), L-carnitine, antithyroid drugs (e.g., methimazole or carbimazole), or by feeding a low-iodine diet (Hill's y/d) (1-3).

Although concurrent use of antithyroid drugs or low-iodine diets does not "interfere" with radioiodine treatment, we recommend that they be discontinued for at least 1-2 weeks before treatment with radioiodine. The main reason stopping the antithyroid drug treatment or the low-iodine diet is to allow the cat to return to a hyperthyroid state by the time the radioiodine treatment is given; this ensures that the cat’s circulating TSH concentrations will be suppressed and that the 131-I uptake by the nonadenomatous (i.e., normal) thyroid tissue and subsequent iatrogenic hypothyroidism will be reduced (12).  Remember that the thyroid adenoma(s) does not need circulating TSH to take up and concentrate the radioiodine.

In contrast, β-blocking agents (e.g., atenolol) or other cardiac medication will not interfere with the radioiodine treatment and can be given up to, and even during, the hospitalization period as needed.

Less commonly, L-carnitine is used to help ameliorate hyperthyroid clinical signs in hyperthyroid cats; the primary effect of this drug is to inhibit the effect of T4 and T3 on the peripheral tissues rather than work on the thyroid gland itself (13). Although L-carnitine may reduce symptoms of hyperthyroidism, it does not change circulating thyroid hormone concentrations or reduce thyroid tumor volume or size. Like the β-blocking agents, L-carnitine does not interfere with the radioiodine treatment and can be given up to, and even during, the hospitalization period.

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. Kintzer PP. Considerations in the treatment of feline hyperthyroidism. Vet Clin North Am Small Anim Pract 1994;24:577-585. 
5. 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.
6. Peterson ME, Melian C, Nichols R: Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. J Am Vet Med Assoc 2001;218:529-536.
7. Wakeling J. Use of thyroid stimulating hormone (TSH) in cats. Can Vet J 2010;51:33-34. 
8. Peterson ME. Diagnostic testing for hyperthyroidism in cats: more than just T4. Journal of Feline Medicine and Surgery 2013:In press. 
9. Trepanier LA. Medical management of hyperthyroidism. Clin Tech Small Anim Pract 2006;21:22-28. 
10. Peterson ME. Treatment of hyperthyroidism and concurrent renal disease: is the "Tapazole trial" necessary? Conference Proceedings 29th Annual Veterinary Medical Forum (American College of Veterinary Internal Medicine) 2011;104-106. 
11. Syme H. A common duo: Hyperthyroidism and chronic kidney disease. NAVC 2013. 2. Syme HM. Cardiovascular and renal manifestations of hyperthyroidism. Vet Clin North Am Small Anim Pract 2007;37:723-743, vi. 
12. Fischetti AJ, Drost WT, DiBartola SP, et al. Effects of methimazole on thyroid gland uptake of 99mTC-pertechnetate in 19 hyperthyroid cats. Vet Radiol Ultrasound 2005;46:267-272. 
13. Peterson ME. Alternative medical treatments for hyperthyroid cats. Conference Proceedings North American Veterinary Conference (NAVC) Conference 2012: Small Animal & Exotics Proceedings 2012;852-858.

My other related blog posts:

• Diagnosing Hyperthyroidism in Cats: Routine Testing Procedures 
• Diagnosis of Hyperthyroidism in Cats: Serum T4 Concentrations 
• Diagnosis of Hyperthyroidism in Cats: Serum T3 Concentrations 
• Diagnosis of Hyperthyroidism in Cats: Serum Free T4 Concentrations 
• Diagnosis of Hyperthyroidism in Cats: Serum Free T4 (Part 2) 
• Diagnosis of Hyperthyroidism in Cats: Serum TSH Concentrations 
• Treatment Options and Considerations for Hyperthyroid Cats 
• Treating Cats with Hyperthyroidism: Antithyroid Drugs 
• How to Dose and Monitor Hyperthyroid Cats on Methimazole 
• Alternative Medical Treatments for Hyperthyroid Cats