This week, I received an email from a veterinarian asking me a question that I frequently get asked. I thought I’d share his question and my response with all of you.
Dear Dr. Peterson,
I have a 12-year-old F/S DSH cat that I diagnosed as being hyperthyroid in June of 2008 because the cat had lost 2.2 lbs. over the year and had a serum T4 level of 4.3 μg/dl (reference range, 0.8 -4.0 μg/dl). The cat was initially placed on methimazole (Tapazole) at the dosage of 2.5 mg, SID. The serum T4 level subsequently dropped to 2.8 μg/dl and the cat maintained her weight.
A year later, however, things started to get interesting. The cat lost another 1.3 lbs. and the serum T4 concentration was back up to 4.3. I increased the methimazole dose to 2.5 mg, BID. When the cat was rechecked in 1 month, her weight was stable, but the serum T4 was still high at 5.4 μg/dl. Over the next 3 months, the cat continued to lose weight and the T4 remained high at 6.1 μg/dl, despite an increase the methimazole dose to 5 mg PO BID. A month later, the T4 value was still high so the dose was again increased to 7.5 mg, BID.
The owner was POSITIVE that the cat was getting the pills. To make sure that wasn’t the problem, I had the owner switch the cat from oral to transdermal methimazole compounded by a local pharmacy. A month later, the cat had lost another 0.4 lbs. and the T4 value had increased 7.2!
This cat is driving me crazy! I am at a loss as to why this cat cannot be stabilized with oral or transdermal methimazole. Any ideas or suggestions?
This scenario isn't really that uncommon in cats treated with methimazole on a long-term basis.
Hyperthyroid cats, as you know, have adenomatous hyperplasia of their thyroid gland. This adenomatous hyperplasia eventually will transform into one or more thyroid adenomas, which continues to grow larger and larger with time. The methimazole does NOT stop this process, since the drug just works to block thyroid hormone secretion. After 1 to 2 or more years, many of these cats will have fairly large palpable goiters and can be difficult to regulate, even with high daily doses of oral or transdermal methimazole.
In some of these cats, the thyroid adenoma also may transform into a thyroid carcinoma as the disease progresses. Again, methimazole does nothing to the tumor pathology and cannot stop this from happening.
So, what do you do? Well, even if you could regulate this cat with 7.5 mg BID now, what are you going to do next year or the year after that when the dose has to go up further? This cat is a relatively young cat, and I believe that the cat should be treated with definitively therapy (surgery or radioiodine) in order to remove the cat's thyroid tumor(s).
This all brings up another important point. We should always tell the cat owner that hyperthyroidism is caused by a thyroid tumor, and that this tumor will grow and grow with time. If methimazole is used as initially treatment, they should be told that the drug may eventually stop working and that other treatments may be needed. Both surgery and radioiodine can be more difficult in cats with large goiters, so early definitive therapy is always best.
In cats that are young to middle-aged, it does not make much sense to me to try to use methimazole on a long-term basis, unless we don't believe that cat will live for another couple of years or the owner refuses definitive therapy.