Thursday, March 21, 2013

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.

  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. 


Electralily said...

Hello! My 10-year-old cat developed symptoms (voracious appetite, weight loss, yowling, panting and spraying outside the litter box), and received radioiodine treatment about 4 weeks ago. I know it may take 3 months or more for thyroid hormone levels to return to normal, but I am wondering about the yowling and spraying. Do these behaviours usually resolve as well, or could these now be habits that will persist? Thanks!

Electralily said...

Hello! My 10-year-old Maine Coon developed symptoms (voracious appetite, weight loss, panting, yowling and spraying outside the litter box) and received radioiodine treatment about 4 weeks ago. I know it may take 3 months for thyroid hormone levels to return to normal, but are the yowling and new litter box behaviours usually resolved as well? Or could they now be habits that will persist? Thanks for all the info you provide in your blog!

Dr. Mark E. Peterson said...

If the behavior is totally due to the hyperthyroidism, the signs should resolve very quickly. Litter box problems are common behavioral issues that have many other causes then hyperthyroidism.

I'd certainly have your veterinarian check a complete urinalysis if the problem is urine spraying or inappropriate urination.

Electralily said...

thanks for the quick response. I haven't seen any panting for a while and Eddie's appetite does seem to be settling down. If the spraying continues I will certainly have a discussion with my vet, as I don't want to continue to have to wash down that room every single day. thanks again.

Melizma Tea said...

Dear Dr. Peterson,

I am very concerned about my 14 year old cat. He was diagnosed in the spring with hyperthyroidism and put on tapazole. In Sept. I brought him back to the vet because I was concerned about how skinny he was. The vet tested him and increased the tapazole dosage. He said not to be concerned unless he was losing weight. My cat's weight held steady, but he was still so skinny that I brought him back two months later - even though he wasn't losing weight, I was not okay with how skinny he was. It turned out he was severely anemic. He had a blood transfusion, and his bone marrow was tested. It looks like it was regenerative,and my vet wants to just wait and redo his CBC to see how he is in another week. Because we aren't sure if the tapazole caused the anemia, he isn't putting him back on it right now. He recommended trying YD. I tried it (just one can) but my cat isn't keen on it, and he is skinnier than ever now so I don't have the luxury of giving it a chance - I don't want to risk him starving to death! He's been off the tapazole for about a month. I am really worried about him - it seems like a catch-22: to get him to gain weight, I have to deal with the hyperthyroidism, but to deal with the hyperthyroidosm it seems I need him to put on weight (e.g. getting him to eat the YD might mean he'd skip some meals until he gave in and started eating it.) To be honest, I'm just not that happy with the way things are going with the vet - to me, time seems to be of the essence, is it not?! It doesn't seem like I have time to wait and see what happens. I am pretty broke but if I have to see a kidney (haha) to get the radioiodine treatment, I will do it, if it will help. My questions are: a) can a cat with anemia or recovering from anemia be a candidate for radioiodine treatment b) can a severely undereweight cat be treated for radioiodine treatment (that's prob. a stupid question but I'm asking anyway!) c) if my cat has been off of tapazole for a month, how much time do we have to get him treated again for his hyperthyroidism? I'm worried he'll die before I can get it properly addressed. d) If I do go for the radioiodine treatment, will he have to wait for a period of time to be treated, since he's eaten a bit of YD? Please help! I want to be proactive and get onto this right away, but I feel my vet isn't really doing that. Thank you so much!!! PS I'm in NYC so I can come to you

Dr. Mark E. Peterson said...

Methimazole can produce hemolytic anemia in cats, If the drug is responsible for the anemia in your cat, it should not be restarted because that would be life-threatening,

We do NOT need a cat on methimazole to do the radioiodine; if fact, we like to have owners stop the drug 1-2 weeks prior to I-131 treatment.

A small amount of y/d will not interfere with the treatment.

Finally, severely underweight cats with hyperthyroidism certainly can be treated with radioiodine.

Obviously, I would need to see your cat and the entire medical record to make the best recommendation, but we would need to be certain that the anemia is resolving before moving forward with radioiodine. Anemia is a sign, not a diagnosis in itself. If the methimazole isn't causing it, we need to figure out what is, since that is a sign of serious disease.

Carolyn Joyce Keller said...

Hi! My 14 year old Siamese cat has lost a significant amount of weight over the past year (originally around 11 lbs and now only 8). My vet tested him in Dec. 2013 for hyperthyroidism, and said he was borderline. His 3T (I think I have this correct) was not really elevated, but his 3T4 was. He had me take a wait and see approach. By April 2014, he still seemed to be losing weight so I had him tested again, with basically the same results. The vet, however, said he felt pretty sure my cat was hyperthyroid, and sent the results to a radiologist who does the radioiodine therapy to see if he was a good candidate. The radiologist said he believed he was a good candidate. After considering the options, i have decided to go ahead with the radioiodine therapy, but I am extremely worried about the isolation period (4 days). My cat
lives with his brother, from whom he has never been separated other than a 1 or 2 day vet visit. Can you provide me with your insight on how well such a cat handles the isolation? He is 14 yet acts very young, but I am very concerned that the isolation may totally stress him out and be too much for him to handle. Any insight is appreciated.

Dr. Mark E. Peterson said...

Almost all hyperthyroid cats we treat do extremely well. That said, we are able to get most of our cats back home to the owners within 3-5 days. And we have large cat condos, ledges and hiding boxes, cat videos and live entertainment (birds or gerbils) for the cats to watch so they don't get bored. We also examine and feed them 3 times a day so they get plenty of human attention.

So I'd talk to your facility and see what they can do before you go ahead. If you aren't happy with the response, you can go someplace else! Siamese cats do tend to be very sensitive and tend to need special care and a bit more attention that most other breeds.

Carolyn Joyce Keller said...

Thanks so much for your response. I did speak with someone from the office that does the therapy, and expressed my concerns. The hospital where the therapy will be done is a spectacular place (Metropolitan Veterinary Associates in Norristown, PA). My golden retriever dog has had both his ACLs repaired there, and also had a cracked molar and abssess corrected there. I have been very, very pleased with the vets and the hospital in general. MVA specializes in emergency vet medicine and more sophisticated vet services. I do trust them, but the woman I spoke with did not say anything about a window for the cats to look out, or videos, etc. like you mentioned. She did say the cats are in a room that is directly across from the doctor's office, but that didn't give me great comfort that it was anything but an enclosed, bunker-type room! I will call and speak with her again. Have you had good results with the radioiodine therapy on older cats? Will my cat actually regain his weight and look healthier? He acts totally normal, but I noticed his fur was sticking out and just not as nice for the past year, and his hind quarters are now so skinny. I am hoping he regains his weight and lives many more years, since he still acts very young. Have you ever experienced anything life-threatening with this therapy, especially in regards to an older cat?

Dr. Mark E. Peterson said...

Over 99% of the cats we treat do well after this treatment. If anything goes wrong, it's not due to the radioiodine but instead related to other medical issues that were sometimes not detected on initial workup.