Q: About four years ago, my then 14-year-old cat was diagnosed with hyperthyroidism (high thyroid hormone levels). She has been on methimazole twice a day since diagnosis, and her thyroid hormone level returned to normal. A few months ago at her checkup, she had started losing weight. Her levels reflected hypothyroidism (low thyroid levels). The blood work was repeated, but again it showed low thyroid levels. The vet wanted to do more blood work and X-rays but for the time being we adjusted (reduced) the methimazole dosage. My cat has gained a little weight, is only nauseous occasionally, eats well but does not eat or drink excessively. Now, as an 18-year-old, she still plays with her toys and loves attention; she even teases my small dog at times. Have you experienced this with any of the cats you have treated? How would you handle it? Or what would you recommend? What could cause it? Thank you in advance for your time and response.
A: There are two potential reasons for your cat’s low thyroid hormone level. One reason could simply be that you were giving your cat an excessive amount of thyroid medication. Sometimes, a cat’s thyroid-medicine requirements will change. This is why the thyroid hormone level must be monitored regularly. I would have handled it the same way you did: I’d reduce the thyroid medication, and see if the thyroid hormone level (and the accompanying clinical signs — losing weight) returned to normal.
The second reason for the low thyroid level would be the presence of another medical condition. When cats are ill in general, their thyroid hormone level will decrease, despite the cat having normal thyroid function. We call this the “euthyroid sick syndrome.” If your cat had a low thyroid hormone level because of a concurrent illness, reducing the thyroid hormone level would not cause the thyroid hormone level to rise back into the normal range. The concurrent illness would still keep the thyroid hormone level depressed.
Because your cat responded to the reduction in thyroid medication (she gained a little weight, is eating well, is active, and heck, she’s still alive four years later!), I think it’s reasonable to conclude that those low thyroid hormone levels were due to you giving her a little too much thyroid medicine. I wish you had measured the thyroid hormone level after you reduced the thyroid dose. If you had, and the level was normal, it would have proven this for certain, but given that your cat is still thriving, I’m sticking with my theory.