The hyperthyroid cat with renal disease (Proceedings)
Hyperthyroidism and chronic kidney disease are two common diseases of older cats. They may occur separately or in the same animal. Because of significant overlap in the clinical presentation it can be difficult to differentiate between the two. The following is a brief discussion of the two diseases, diagnosis and treatment as well as recommendations on the identification and treatment of cats with chronic kidney disease and hyperthyroidism.
Common historical findings include weight loss, vomiting, diarrhea, increased water consumption and increased urination. With hyperthyroidism the appetite and activity tend to be increased whereas cats with chronic kidney disease have normal to decreased appetites and activity levels. Apathetic hyperthyroidism has been described as a rare, likely more advanced, stage of hyperthyroidism in which the appetite may be depressed and the cat is more lethargic.On clinical evaluation cats with hyperthyroidism and chronic kidney disease may have a poor hair coat, skin tent, generalized muscle wasting, tacky mucous membranes, tachycardia, a heart murmur, hypertension, and weakness. Cats with chronic kidney disease may have bilaterally palpable small kidneys or one small and one larger kidney. The kidneys in cats with hyperthyroidism should be normal in size but some cats with concurrent early kidney disease may have small kidneys. Almost all cats with hyperthyroidism have a palpable nodule. Unfortunately we have all tested cats with 'thyroid nodules' only to find they were euthyroid.
On laboratory evaluation, the CBC may be normal with both conditions but anemia is common with more advanced forms of chronic kidney disease. Elevations in liver enzymes are common with hyperthyroidism and not seen with chronic kidney disease. Azotemia can be seen with both conditions. In hyperthyroidism, azotemia may be prerenal or they may have concurrent chronic kidney disease. Electrolyte abnormalities (hyperphosphatemia and hypokalemia) are more specific for chronic kidney disease. Urine concentrating ability may be decreased in both disorders. Repeatable isosthenuria (s.g. 1.008 – 1.012) is a feature of kidney disease whereas with hyperthyroidism urine specific gravity can be variable but is usually greater than isosthenuric. Problematic is that with early kidney disease urine may not be isosthenuric as there may be loss of nephrons but enough remaining functional nephrons to concentrate urine to some degree.
The diagnosis of hyperthyroidism is typically made based on the historical, physical and clinical findings mentioned previously in addition to evaluation of serum total thyroxine (TT4) levels. Serum TT4, diagnostic in about 90% of cats with hyperthyroidism, can be affected by the quantity of carrier proteins, alterations in metabolism, the ability to transport thyroxine into cells, and binding of T4 within the cells. These are the mechanisms that may alter TT4 levels in non-thyroidal illness and are the basis for the recommendation of measurement of free T4. Free T4 is what is available for entry into cells so may be a more accurate reflection of thyroid gland function. Tests using equilibrium dialysis (ed) are recommended because other methodologies usually underestimate free T4. A free T4 is able to identify 95% of hyperthyroid cats with a TT4 within the reference range. This test is more sensitive than TT4 for diagnosing hyperthyroidism but is also less specific. This decreased specificity is why it is not recommended as a screening test. In fact in a study looking at FT4 (ed) in sick cats there were some euthyroid cats that were severely ill with elevated FT4 (ed). So illness, including chronic kidney disease, could decrease measureable TT4 (masking hyperthyroidism) and increase FT4 (ed) leading to a misdiagnosis if hyperthyroidism.