Q & A: Renal Disease in a Newly Diagnosed Hyperthyroid Cat

I have a 14-year-old, male DSH cat that I diagnosed recently with hyperthyroidism (serum T4 value = 10.9 μg/dl; reference range, 0.8-4.0 μg/dl). However, this cat has a 5-year history of clinical signs consistent with hyperthyroidism. The cat used to weigh 8.0 kg and now is only 2.6 kg!

The cat is now showing all of the classical clinical features of hyperthyroidism, including weight loss despite an increased appetite, vomiting, diarrhea, and polyuria with polydipsia. On physical examination, he has a very large left thyroid lobe, a heart murmur, and severe hypertension.

Serum chemistry analysis revealed a high urea nitrogen (62 mg/dl) with a normal creatinine (1.5 mg/dl). The urine specific gravity is 1.020.

I'm worried about concurrent renal disease in this cat. I've had poor luck treating severe, long-standing hyperthyroid cats like this one. Some of the other cats with concurrent hyperthyroidism and kidney disease that I've cared for in the past developed renal failure weeks after treating with low dose methimazole. Despite stopping the drug and starting fluid therapy, I couldn't reverse the renal azotemia in some of those cats.

So, do you have a suggestion for handling hyperthyroid cats like this?

Thank you very much.

My Response:

First of all, it is certainly clear that this cat as chronic kidney disease (CKD). The serum urea nitrogen concentration is about twice the upper limit of the reference range.

Although the serum creatinine is normal, it's very likely that the creatinine value would be quite high if the cat didn't have muscle wasting associated with the hyperthyroidism. (Remember, creatinine is made in muscle tissue.) In addition, cats with moderate to severe hyperthyroidism will develop an increased renal blood flow and glomerular filtration rate (GFR), which could lower the circulating levels of both urea nitrogen and creatinine (1,2).

Although it is difficult to classify the stage of CKD in this cat, I'd predict that he probably has Stage 3 CKD (see this link for more more information on IRIS staging of renal disease in cats). Based on the length of time he probably has been hyperthyroid, he could even be in Stage 4 CKD.

So how would I handle this cat? First of all, I would control all of the possible contributing factors before you do too much to treat the hyperthyroidism. For instance, I would definitely start amlodipine to control the cat's hypertension (2,3). I would also culture the cat's urine and treat any urinary tract infection that may be present. I'd check a urine protein:creatinine ratio to evaluate for proteinuria, which may need to be treated with an ACE inhibitor. Finally, I'd start the cat on a renal diet to find a food that he will eat; diet is the mainstay of management of CKD in cats with renal disease.

Once we get those treatments underway, then you can start low-dose methimazole treatment (1.25 mg once a day, initially). Because this will lower the the renal blood flow and GFR to where the values should be (right now they are artificially high, even for a normal cat), it is the azotemia will worsen. You may want to start the cat on subcutaneous fluids (100 ml per day or so, depending on cardiac status) to help maintain hydration and increase renal blood flow. I would check the serum urea nitrogen and creatinine every 3 to 5 days during the initial treatment, and make dose adjustments as needed.

If the azotemia worsens dramatically on methimazole, you might NOT be able to treat the cat's hyperthyroidism. Remember, however, that the hyperthyroid state is likely contributing to this cat's kidney disease through the development of systemic and glomerular hypertension (4,5). In addition, this cat will soon die because of the hyperthyroidism if we don't try to get it under control.

The Bottom Line: Because control of the hyperthyroidism will reverse the glomerular hypertension and slow down progression of renal disease, I believe that we should try to treat all hyperthyroid cats with renal disease. In those cats with severe underlying renal disease whose azotemia dramatically worsens after methimazole, continued treatment of the hyperthyroidism might not be possible. In those cats, however, the long-term prognosis is very poor to grave.

References: 

1. Langston CE, Reine NJ. Hyperthyroidism and the kidney. Clinical Techniques in Small Animal Practice 2006;21:17-21.
2. Syme HM. Cardiovascular and renal manifestations of hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:723-743.
3. Stepien RL. Feline systemic hypertension: Diagnosis and management. Journal of Feline Medicine and Surgery 2011;13:35-43.
4. 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.
5. 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.