Sunday, October 23, 2011

Treating Cats with Hyperthyroidism: Antithyroid Drugs

In cats, hyper­thyroidism can be treated in four ways — chronic administration of an antithy­roid drug, surgical thyroidectomy, radioactive io­dine (131-I), or lifelong feeding of an ultra-low iodine diet.

The treatment of choice for an individual cat depends on several factors, including the age of the cat, presence of associated heart or kidney dis­eases or other major medical problems, availability of a skilled surgeon or radioiodine treatment facility, and owner's preference (1-4).

In this post, I’m going to discuss the use of antithyroid drugs for treating cats with hyperthyroidism. This is the most common means that veterinarians use to treat this common condition, so let’s start by discussing the pros and cons of this form of treatment.

The Advantages and Disadvantages of Antithyroid Drugs

Advantages
Chronic management with antithyroid drugs is a practical treatment option for many cats with hyperthyroidism, and offers many advantages. Medical management requires no special facilities and can be prescribed by all veterinarians (1-4). These drugs cause a rapid fall in serum thyroid hormone levels (i.e., the high serum T4 normalizes within 1-3 weeks), which may be desirable in severely affected hyperthyroid cats (5,6).

Anesthesia is avoided, as are the surgical complications associated with thyroidectomy (I’ll be discussing surgical treatment in my next post). In contrast to surgery or radioiodine treatment, hospitalization is not required with medical treatment. Finally, the initial, upfront costs of antithyroid drugs is much less than with either surgical or radioactive iodine treatment.

Disadvantages
Long-term medial management also has many disadvantages. This form of treatment is not curative, is highly dependent on owner and cat compliance, and requires regular biochemical monitoring to ensure the efficacy of treatment (1-4). Side effects are common, occurring in up to 20% of cats (1-6). Even though the initial cost of medical treatment may be far less initially, the cost of ongoing monitoring over a period of months to years can exceed that of thyroidectomy or radioiodine therapy.

These antithyroid drugs also come with other drawbacks. Since they block thyroid hormone synthesis but do not destroy the cat’s thyroid tumor, these drugs never cure the hyperthyroidism and relapse will always occur if daily medication is discontinued (1-6). Most importantly, the benign thyroid tumor — which is present in all cats with hyperthyroidism (7,8) — continues to grow and, after many months, may transform from adenoma to thyroid carcinoma in some cats (9).

Long-term medical management is best reserved for cats of advanced age or for those with concurrent diseases, and for when owners refuse either surgery or radioactive iodine. In addition to long-term treatment, medical management is also advised prior to surgical thyroidectomy to decrease the metabolic and cardiac complications associated with hyperthyroidism. Short-term medical management is often recommended as trial therapy to determine the effect of restoring euthyroidism on kidney function, especially in cats with suspected chronic kidney disease (1-4,10).

Methimazole and Carbimazole: The 2 Antithyroid Drugs

The two drugs methimazole and carbimazole are commonly recommended for managing cats with hyperthyroidism (1-6). A related drug, propylthiouracil, often used in human medicine, is not recommended for cats because of a high incidence of serious adverse reactions —especially anemia and bleeding problems (11).

Methimazole blocks thyroid hormone synthesis by inhibiting thyroid peroxidase, an enzyme involved in the oxidation of iodide to iodine, incorporation of iodine into thyroglobulin, and coupling of tyrosine residues to form T4 and T3 (12). Methimazole does not block the release of preformed thyroid hormone, so there is a delay of 1 to 3 weeks before serum T4 concentrations return to normal after initializing therapy (5, 6).

Carbimazole is a pro-drug of methimazole. That means that after oral administration of carbimazole, it is almost immediately converted to methimazole by the cat (13). So basically, it turns out that carbimazole and methimazole end up being the same drug (1-4).

Different Drug Formulations of Methimazole & Carbimazole

Methimazole Tablets
Methimazole is specifically licensed for treatment of feline hyperthyroidism both in the USA and Europe as 2.5- and 5-mg tablets (Felimazole, Dechra Veterinary Products). It is also available as a generic and brand name drug for human use (Tapazole). For most hyperthyroid cats, a starting dose of 1.25 mg to 2.5 mg methimazole is administered twice daily is recommended (1-4,14).

Carbimazole Tablets
Carbimazole is available for human use in many European countries (brand name, NeoMercazole), Australia and Japan (2-4). This drug is not available as a licensed drug in the USA, but it is available though compounding pharmacies.

As noted above, carbimazole exerts its antithyroid effect through immediate conversion to methimazole when administered orally (13). Serum concentrations of methimazole achieved after carbimazole administration are less than after a similar weight of methimazole such that a 5-mg dose of carbimazole is approximately equal to 3 mg of methimazole (3,6). Because of that, a starting dose of regular carbimazole of 2.5 mg to 5 mg twice daily is commonly recommended for restoring euthyroidism (2-4,6).

Carbimazole is often touted as having a lower incidence of adverse reactions such as vomiting and anorexia (6,15). This may be because it is tasteless whereas methimazole has a bitter taste (3,6). However, Felimazole, as licensed for veterinary use, is sugar-coated; provided the tablet is not crushed, the bitter taste is presumably avoided.

Carbimazole Tablets (Controlled-Release)
A controlled-release formulation of carbimazole (Vidalta, Intervet Schering Plough) is licensed for cats in Europe for once daily administration (16,17). This formulation is not available in the USA.

Administration of this drug with food significantly enhances its absorption (16). The starting dose for controlled release carbimazole is 15 mg administered once daily. In cats with mild hyperthyroidism (total T4 concentration <100 nmol/L or < 8 μg/dl), a 10 mg once daily is recommended (2-4,16).

Transdermal Antithyroid Drugs
Carbimazole and methimazole can be reformulated by a veterinary compounding pharmacy and applied to the non-haired inner portion of a cat’s pinnae (ear lobe) for transdermal administration (18-20). Such custom formulation increases expense of therapy and the stability of the product can never be guaranteed.  To prevent absorption of the drug through one's own skin, it is best to wear gloves or a finger cot for application, and wash your hands afterwards.

Both antithyroid drugs are generally effective in cats when administered at a dose of 1.25 mg to 2.5 mg twice daily transdermally (2-4).  One advantage of using a compounded formulation of methimazole (or carbimazole) over the 2.5- or 5-mg tablets is that it is easier to make smaller or finer dose adjustments.

Transdermal administration is associated with fewer gastrointestinal side effects than the oral route (19,21,22), but some cats resent manipulation of their ears and crusting can occur between doses leading to erythema. These problems can usually be prevented by removing any crusted material and cleaning the ear flap prior to administration.

Monitoring of Hyperthyroid Cats on Antithyroid Drug Treatment

Initial doses of the antithyroid drug vary depending on the cat’s pretreatment serum T4 value and goiter size (i.e., size of the thyroid tumor). In general, however, most cats are started on 1.25-2.5 mg of methimazole or 2.5-5 mg of carbimazole, both administered twice daily (1-4,21-23).

Initial Monitoring
Initially, cats should be reassessed after 2 to 3 weeks and a serum total T4 concentration measured. When monitoring, time of serum T4 sampling in relation to the administration of the antithyroid drug is not overly important (1-4,24). The goal of medical therapy is to maintain total T4 concentrations within the lower half of the reference range (1-4). Low serum T4 values should be avoided, however, because it has been shown that chronic hypothyroidism is deleterious to kidney function and may worsen already present chronic kidney disease (25,26).

If hyperthyroidism persists during antithyroid drug treatment, lack of owner or cat compliance should always first be eliminated as a reason for the failure of therapy. If the serum T4 concentrations remain high with proper treatment, however, the daily dose of methimazole or carbimazole can be increased in 2.5-mg increments, reassessing the cat again in 3 to 4 weeks (1-4).

Long-Term Monitoring and Treatment
For chronic management (once euthyroidism has been achieved), the daily antithyroid drug dosage is adjusted to the lowest possible dose that effectively maintains euthyroidism. Once the dosage has stabilized, the cat should be monitored every 3 to 6 months and as needed clinically. At time of each of these rechecks, a complete physical examination should be perform together with determination of a complete blood count, serum chemistry profile, and serum T4 concentration.

Relapses are common in cats treated with an antithyroid drug. Some cats will become more difficult to medicate over time, whereas others will need higher daily drug dosages to inhibit thyroid hormone secretion as their thyroid tumors continue to grow larger and larger (27).

It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values. For example, if the T4 reference range is listed as 0.8-4.0 μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 μg/dl (20-32 nmol/L). Recent research indicates that hyperthyroidism may contribute to the development or progression of chronic renal disease in cats (28-30).  Leaving a hyperthyroid cat untreated (or poorly regulated with methimazole or carbimazole) may therefore be detrimental to long-term kidney function and is never recommended.

During long-term treatment, it is again important to avoid inducing hypothyroidism, which may be deleterious to the cat’s kidney function (25,26). If hypothyroidism is suspected (which can develop even if the T4 is low-normal), a complete thyroid panel is recommended, including determination of the serum concentrations of total T4, free T4, T3, and TSH (see my previous blog posts on diagnostic testing for more information about these tests). The findings of low serum free T4 with high TSH concentrations is diagnostic for iatrogenic hypothyroidism; in those cats, the daily dose of methimazole  should be decreased.

Because antithyroid medications have no effect on the underlying lesion, the thyroid nodules continue to grow larger and larger over time. This may necessitate an increased daily dose with time. In some cats, large enough dosages can no longer be administered to control the hyperthyroidism and surgery or radioiodine is needed to control the hyperthyroidism (27).

Side Effects & Adverse Reactions

Most clinical adverse reactions occur within the first 3 months of therapy (1-4). Mild clinical side effects of vomiting, anorexia, or depression occur in approximately 10-15% of cats, usually within the first 3 weeks of therapy (4,5). In most cats, these reactions are transient and do not require permanent drug withdrawal.

Mild Side Effects
Figure 1: Facial excoriations
due to methimazole
Early in the course of drug therapy, mild and transient hematological abnormalities, including leucopenia (low total white blood cell count), lymphocytosis (high lymphocyte count), or eosinophilia (high eosinophil count) develop in up to 15% of cats without any apparent clinical effect (1-6).

Self-induced excoriations of the head and neck (from scratching) occasionally develop, usually within the first 6 weeks of therapy (see Figure 1).

Less commonly, generalized enlargement of lymph nodes may develop during drug treatment (31). If either of these adverse effects occur, the drug must be stopped and another form of treatment given.

Life Threatening Side Effects
Figure 2: Bleeding from gums due
to methimazole
More serious hematological complications occur in less than 5% of cats and include a severe lowering of the white blood cell count (agranulocytosis) or platelet count (thrombocytopenia).  Liver dysfunction, characterized by marked increases in all hepatic enzymes, develops in less than 2% of cats (1-5).

Rarely, cats may also develop a severe bleeding tendency during drug treatment (see Figure 2) (5,32). All of these adverse effects are reversible upon discontinuation of the medication.

The Bottom Line

Short-term or chronic management with antithyroid drugs provides a useful treatment option for many cats with hyperthyroidism. However, this treatment does not cure the disease and requires daily medication for the rest of the cat's life.  Like any prescription drug, methimazole or carbimazole can produce adverse side effects, which may be life-threatening is some cats. For all of these reasons, antithyroid drugs will never be the "treatment of choice" for all cats with hyperthyroidism.

References
  1. Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Barcelona, Multimedica, 2008; 127-168.
  2. Baral R, Peterson ME. Thyroid diseases. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; in press.
  3. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press.
  4. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
  5. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150–157. 
  6. Mooney CT, Thoday KL, Doxey DL. Carbimazole therapy of feline hyperthyroidism. Journal of Small Animal Practice 1992;33:228–235. 
  7. Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Veterinary Clinics of North America Small Animal Practice 1994;24:541-565.
  8. Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645.
  9. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.
  10. Becker TJ, Graves TK, Kruger JM, et al. Effects of methimazole on renal function in cats with hyperthyroidism. Journal of the American Animal Hospital Association 2000;36:215–223. 
  11. Peterson ME, Hurvitz AI, Leib MS, Cavanagh PG, Dutton RE. Propylthiouracil-associated hemolytic anemia, thrombocytopenia, and antinuclear antibodies in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1984;184:806-808. 
  12. Cooper DS. Antithyroid drugs. New England Journal of Medicine 2005;352:905-917.
  13. Peterson ME, Aucoin DP. Comparison of the disposition of carbimazole and methimazole in clinically normal cats. Research in Veterinary Science 1993;54:351–355. 
  14. Plumb DC. Plumb's Veterinary Drug Handbook (7th Ed). PharmaVet Inc, Stockholm, Wisconsin 2011.
  15. Bucknell DG. Feline hyperthyroidism: spectrum of clinical presentions and response to carbimazole therapy. Australian Veterinary Journal 2000;78:462-465. 
  16. Frénais R, Burgaud S, Horspool LJ. Pharmacokinetics of controlled-release carbimazole tablets support once daily dosing in cats. Journal of Veterinary Pharmacology and Therapeutics 2008;31:213-219.
  17. Frenais R, Rosenberg D, Burgaud S, et al. Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism. Journal of Small Animal Practice 2009;50:510-515. 
  18. Hoffman S, Yoder A, Trepanier L. Bioavailability of transdermal methimazole in a pluronic lecithin organogel (PLO) in healthy cats. Journal of Veterinary Pharmacology and Therapeutics  2002;25:189-193. 
  19. Hoffman SB, Marks SL, Taboada J et al. Transdermal methimazole treatment in cats with hyperthyroidism. Journal of Feline Medicine and Surgery 2003;5:77–82. 
  20. Buijtels JJ, Kurvers IA, Galac S et al. Transdermal carbimazole for the treatment of feline hyperthyroidism, Tijdschrift voor Diergeneeskunde 2006;131:478-482. 
  21. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:775-788. 
  22. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22-28. 
  23. Trepanier LA, Hoffman SB, Knoll M, et al. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 2003;222:954–958. 
  24. Rutland BE, Nachreiner RF, Kruger JM. Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats. Journal of Veterinary Internal Medicine 2009;23:1025-1030. 
  25. Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. Journal of Veterinary Internal Medicine 2010;24:863-869. 
  26. Williams T, Elliott J, Syme H. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine 2010;24:1086-1092. 
  27. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
  28. Lapointe C, Bélanger MC, Dunn M, et al. N-acetyl-beta-D-glucosaminidase index as an early biomarker for chronic kidney disease in cats with hyperthyroidism.  Journal of Veterinary Internal Medicine 2008;22:1103-1110. 
  29. van Hoek I, Lefebvre HP, Peremans K, et al. Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine. Domestic Animal Endocrinology 2009;36:45-56.  
  30. van Hoek I, Meyer E, Duchateau L, et al. Retinol-binding protein in serum and urine of hyperthyroid cats before and after treatment with radioiodine. Journal of Veterinary Internal Medicine 2009;23:1031-1037. 
  31. Niessen SJ, Voyce MJ, de Villiers L, et al. Generalised lymphadenomegaly associated with methimazole treatment in a hyperthyroid cat. Journal of Small Animal Practice 2007;48:165-168. 
  32. Randolph JF, DeMarco J, Center SA, et al. Prothrombin, activated partial thromboplastin, and proteins induced by vitamin K absence or antagonists clotting times in 20 hyperthyroid cats before and after methimazole treatment. Journal of Veterinary Internal Medicine 2000;14:56-59. 

24 comments:

Anonymous said...

I agree completely!

jennyk said...

Dr Peterson, you say that the starting dose for 'regular' carbimazole is 5 to 10mg a day but for slow-release carbimazole it is 15mg a day. Is that because not all the slow-release form is released and absorbed by the cat or is there another reason?

Dr. Mark E. Peterson said...

The absorption kinetics of the slow-release carbimazole preparation is different from the "regular" carbimazole preparation. As it states in the name, the slow release formulation released more slowly into the bloodstream. And that affects the dosage needed to block T4 secretion by the cats' thyroid tumors.

jennyk said...

Thank you!

K said...

Can you please share your thoughts on whether topical carbimazole might be better tolerated than topical methimazole. I have sibling cats who are hyperthyroid. They have side effects from Methimazole at low doses (1mg/1.5mg). I am in the process of obtaining a more dilute compounded gel so I can start again in a few days with a .5mg dose. But if you think I might have better luck with carbimazole, I will try to locate some.

Many thanks for your helpful blog =)

Dr. Mark E. Peterson said...

If your cats are showing Gi effects to the transdermal methimazole, it's unlikely that a switch to carbimazole would help. Once absorbed, carbimazole must be converted to methimazole in the liver to work, so basically, it's the same drug.

When given orally, methimazole has a bitter taste, whereas carbimazole is fairly tasteless. I believe that's why we see less GI side effect with oral carbimazole. But once we by-pass the GIT, the taste of the drugs isn't a factor.

K said...

Thanks for your response. I was hoping it would be otherwise. I will still try carbimazole, because the cats want clean their ears after I apply the Methimazole. At the very least, switching to carbimazole compounded to the lowest available should help me start with a very low dose and gradually increase. Thanks again for your help. Keep up the good work. =)

Dr. Mark E. Peterson said...

Let me just say this to clear — I have not ever used transdermal carbimazole so I can't say that it won't work better than the transdermal methimazole. But if it is better tolerated, I would be a bit surprised ... keep me posted.

Dawn Eaton said...

Hi. I have a 13 year old cat and she was just diagnosed with hyperthyroidism, and her count or level was/is 16. I understand from reading on your website, that this is very bad. The vet has put her on tapazole 5mg 3 times daily. Is that a normal amount? Also, she has only been on it for a week and she is exhibiting signs of that facial itching, not good, especially her ears. I think I am going to go the iodine route, I hope her level is not too high for that? Thank you for any insight you can give.

Dr. Mark E. Peterson said...

A serum T4 concentration of 16 µg/dl is higher than the average T4, but the value is not extremely high. Most cats do not need 15 mg of methimazole per day, but the dosage needed is quite variable. Most cats with a T4 of 16 µ/dl can be controlled on daily doses of 5-7.5 mg.

The itching is an allergic reaction. You can lower the dose to see if that helps, but in most cats that side effect is not dose related.

Radioiodine should be a good treatment for your cat, especially if all of her other body functions are good (no kidney disease, etc).

Jenn K said...

Hi Dr. Peterson! I came upon your site just by happenstance. My 9 yr. old male has Hyperthyroidism and has had it since probably October 2013. I put him on Methimizole 1/2 bid. He was doing great on this dose and his values went from 7.5 down to 3 or 4 (or so). He unfortunately now has liver failure due to the meds. This makes me VERY angry and frustrated, and I have tried homeopathy for him, but in reading everything, the tumor can grow. (I guess I had forgotten about this).

Anyway, I am considering radioactive iodine therapy. Should I do this w/my cat that has liver failure? He doesn't look jaundice any longer as I am using homeopathy on him for liver failure, but what are your thoughts? Cost, how long do the results last? Can the tumor grow back?

Thank you SO much for any input you can provide!!

Dr. Mark E. Peterson said...

The liver failure should be reversible once the methimazole is stopped.

If severe liver failure continues, then you need an ultrasound and liver biopsy to diagnosed the underlying problem and determine the best course of treatment.

Jenn K said...

Thanks so much for your response. I just found out that the course of tx here is $1275!! Unfortunately, I cannot afford that... :-(

Cristina Samaniego said...

Dr. Peterson,

My 13 yr. old cat was just diagnosed with hyperthyroidism-value of 13- and was prescribed 5mg/once daily of Felimazole. My concern is I am leaving town early next week and will be gone for 9 days. Though I have someone coming to check on her, it won't be daily, so do I start her on the regimen now, knowing she won't receive it 4-5 days while I'm gone or wait until I get back? I haven't started her on the Felimazole yet since I just picked it up this evening. I'm worried about both the side effects and not being here to catch them and also any rebound effects from starting, stopping, and then starting the meds again. I feel the latter is more dangerous, but with her high values, I'm also hesitant to hold off the medication.

Any insight to pros and cons would be greatly appreciated.

Dr. Mark E. Peterson said...

Methimazole (Felimazole) takes a few days to start to inhibit thyroid hormone secretion from your cat's thyroid tumor. Then if you stop giving the drug, the effects of the drug quickly wear off, and the serum thyroid values will rise again into the hyperthyroid range.

I don't see any reason or advantage of starting the Felimazole until you get back from your trip. If you travel frequently, you should consider definitive treatment for the thyroid tumor so you don't have to worry about this.

Remember that all of these cats have a thyroid tumor and most of these thyroid tumors will continue to grow slowly with time. At 13-years of age, your cat hopefully will live a long time, giving the thyroid tumor a long time to grow.

Jo said...

Dr. Peterson,
My 19 yr old cat with hyperT (for a 2nd time following I131 7 yrs ago!), as well as Stage 3 CKD, has been on 1.25mg Felimazole since last Oct. His Jan T4 & FT4 levels were a little less than mid-normal range, so his hyperT appeared to be well controlled.

We retested this month & although his T4 is the same as in Jan (1.8), his FT4 has jumped to 74! My vet & I were very surprised to see such a significant difference in the two numbers! The only difference in the Jan & April tests is that blood was drawn about 2.5hrs after his morning Felimazole dose in Jan, whereas it was drawn about 7 hrs after his am Felimazole dose for the April test.

Is this therefore evidence that the 1.25mg dose was, in fact, NOT controlling hormone production for an adequate length of time before the next dose as the Jan tests led us to believe? Is it, therefore, better to draw blood several hrs after a Felimazole dose to accurately determine if hormone production is being adequately controlled on a consistent, ongoing basis?

We are increasing my cat's dose to 2.5mg BID as a result of the latest test results. Given the dramatic differences between the T4 & FT4 levels, I hope most vets are running FT4 tests along with T4 when monitoring cats on longterm drug treatment!

Thank you for all you do, Dr. Peterson!

Jo

Dr. Mark E. Peterson said...

These last results do not make a lot of sense, especially if you thought your cat is doing well. I suspect that the T4 or FT4 is wrong, but I can't tell which one. It is possible that the CKD is suppressing the total T4 more than the free T4 but if that's the case, I'd expect your cat to be clinically sick (not eating well, etc).

If possible, I'd like to do a whole panel, including a total T4, T3, free T4 (by dialysis) and TSH. Then we can look at the whole panel to determine what dose adjustments to make.

I would not have doubled the methimazole dose based on a T4 of 1.8. This will likely lead to hypothyroidism and could lead to worsening of the renal disease.

Talk to your vet about all of this. You should recheck within a week of any dose change.

Dr. Mark E. Peterson said...

These last results do not make a lot of sense, especially if you thought your cat is doing well. I suspect that the T4 or FT4 is wrong, but I can't tell which one. It is possible that the CKD is suppressing the total T4 more than the free T4 but if that's the case, I'd expect your cat to be clinically sick (not eating well, etc).

If possible, I'd like to do a whole panel, including a total T4, T3, free T4 (by dialysis) and TSH. Then we can look at the whole panel to determine what dose adjustments to make.

I would not have doubled the methimazole dose based on a T4 of 1.8. This will likely lead to hypothyroidism and could lead to worsening of the renal disease.

Talk to your vet about all of this. You should recheck within a week of any dose change.

LynnT said...

Dr. Peterson,

My 18 yr old cat was diagnosed in mid-February with hyperthyroidism & began 5 mg Tapazole immediately.

A week later he had surgery to remove a tumor from his upper left left. They shaved a small area on his neck & both legs. I assume blood work & IV's.

Shortly thereafter he began scratching his neck on the left side where he had been shaved.

His T-4 was retested in March & was in normal range.

Just recently his scratching has become a problem, a lot of fur is now missing on the left side of his neck & behind his left ear. He also had some small sores on the neck.

The vet's office gave me Derma-Vet ointment, saying that it was due to shaver burn. The ointment seemed to make matters worse, I was finding hair all over the house & the bald area on his neck became larger. I stopped the ointment yesterday.

I see that they can have this type of allergic reaction to Tapazole.

What is the protocol when this happens?

Am I correct in assuming that a lower dose does will not resolve this?

Any insight is appreciated.

Thank you

Dr. Mark E. Peterson said...

It certainly sounds like a reaction from the methimazole. The only way to know for certain is to stop administering the drug for at least 2 weeks - if the scratching and sores improve, then it is likely that it's due to the methimazole.

In that case, you could always try giving the methimazole again to see if the sores recur; if they do, another treatment has to be used. This side effect is not dose related so a lower dose will not help.

LynnT said...

Dr Peterson,

Thank you for your fast reply.

I have a couple more questions, How soon after starting Tapazole does a allergic reaction typically appear?

How soon after stopping the Tapazole should the scratching stop?

Thank you once again for your help.

Dr. Mark E. Peterson said...

Most cats that develop an allergic reaction will do so within the first 6 weeks, but this can develop after many weeks in a few cats.

If an allergic reaction develops, most cats will decrease the amount of scratching within 2-3 days but it may take 2-4 weeks for the lesions to heal. Generally, a notable improvement is observed within 7-10 days.

cat lady said...

I have a 14 year old cat whose been treated for a couple of years with methimazole 2.5 mg bid. She recently has lost weight. Is there another medication? I know about the other treatment options, but not able financially. Have you heard of iopanoic acid?

Dr. Mark E. Peterson said...

see this blog post:

http://animalendocrine.blogspot.com/2012/09/alternative-medical-treatments-for.html