Managing Feline Hyperthyroidism in Patients with Renal Insufficiency
Sponsored by Dechra Veterinary Products
Hyperthyroidism and chronic kidney disease (CKD) are 2 of the most common conditions seen in cats. Hyperthyroidism is the most common endocrine disorder of older cats, affecting 1.5% to 11.4% worldwide,1 and 30% of cats >15 years of age have CKD.2 Thus, clinicians often need to manage cats with both conditions. A common belief is that hyperthyroid cats with CKD should be kept slightly hyperthyroid to improve kidney function,1 but is this really true?
Tiggie’s Case
Tiggie, a 15-year-old spayed female domestic shorthair cat, was presented for a wellness examination. Tiggie had lost 1.5 lb (0.7 kg) since presentation 1 year prior. Tachycardia, a grade III/VI murmur, and thyroid slip were noted.
Blood work revealed elevated total thyroxine (T4; 5 µg/dL; range, 0.8-4.7 µg/dL) and ALT (121 U/L; range, 25-97 U/L). BUN, creatinine, and symmetric dimethylarginine (SDMA) were all within range. Urinalysis was normal, aside from a mildly dilute urine specific gravity (USG) of 1.024.
Tiggie was diagnosed with hyperthyroidism and started on methimazole (2.5 mg every 12 hours). The murmur was suspected to be secondary to hyperthyroidism, and a cardiac investigation was recommended.
Laboratory tests were repeated 4 weeks after initial presentation, and total T4 (2.4 µg/dL) and ALT (90 U/L) were within range. Mild azotemia (BUN, 42 mg/dL; range, 19-34 mg/dL: creatinine, 2.5 mg/dL; range, 0.9-2.2 mg/dL) was observed, and SDMA was mildly elevated (19 µg/dL; range, 0-14 µg/dL). Urinalysis was unremarkable, apart from remaining mildly dilute with a USG of 1.021. Urine protein:creatinine ratio and blood pressure were normal.
Tiggie was diagnosed with International Renal Interest Society (IRIS) stage II, nonproteinuric, normotensive CKD3 and was started on a renal support diet and kept on the same dose of methimazole.
Why Did Azotemia Develop After Treatment?
In one study, 15.3% of cats that were nonazotemic at diagnosis of hyperthyroidism became azotemic within 240 days.4 Hyperthyroidism masks azotemia; cachexia causes muscle loss and lowers creatinine levels while increased glomerular filtration rate (GFR) raises cardiac output and lowers BUN, creatinine, and SDMA levels.5
Can We Predict Which Cats Will Become Azotemic?
Unfortunately, evaluations of creatinine, BUN, and USG before initiation of treatment are not reliable predictors of which cats will develop azotemia. Similarly, high pretreatment SDMA (>14 µg/dL) can only identify one-third of cats that become azotemic after treatment.6
Does Hyperthyroidism Support Renal Function?
A common misconception is that increased GFR due to high thyroid levels supports renal function. However, hyperthyroidism activates the renin-angiotensin-aldosterone system, therefore increasing cardiac output and renal hypertension and exacerbating CKD.1
The target total T4 range for cats without CKD is 1 to 2.5 µg/dL (range, 1-4 µg/dL). Although clinical resolution of hyperthyroidism is still the goal for cats with CKD, target T4 should be in the upper half of the reference range.1
What Is the Prognosis for Cats with Hyperthyroidism & CKD?
Hyperthyroid cats without CKD have a median survival time of ≤5.3 years.1 The prognosis for cats with CKD and hyperthyroidism is affected by many factors, but one study found the median survival time for hyperthyroid cats that developed azotemia to be 417 days.4
Conclusion
Hyperthyroidism and CKD often occur together in older cats. Clinicians should feel comfortable managing these complicated cases and ensuring that target T4 levels are maintained to support kidney function.
