Hyperthyroidism and chronic kidney disease (CKD) are both common disorders in older cats. Therefore, it should not be surprising that both disorders frequently develop together in the same cat. The prevalence of concurrent renal disease in cats with hyperthyroidism is estimated to be approximately 30-40%.Hyperthyroidism tends to "artificially" increase the renal blood flow (RBF), as well as the glomerular filtration rate (GFR). When the GFR is increased in a hyperthyroid cat with underlying CKD, it can mask renal insufficiency; serum concentrations of urea nitrogen and creatinine may be normal despite mild to moderate kidney disease. Decreased muscle mass, a common feature of hyperthyroidism, also contributes to the lowered serum creatinine level in these cats (since creatinine is derived from muscle tissue).Successful treatment of hyperthyroidism restores the high serum T4 concentration to normal and, in cats without CKD, also returns the high GFR back to normal values. In cats with CKD, however, the GFR will fall to the low-normal or subnormal levels expected with moderate renal dysfunction. Therefore, this decrease in GFR can result in the apparent worsening of the serum kidney function tests or the development of renal disease.1"7 It is important to remember, however, that treating the hyperthyroidism itself does not cause the CKD in these cats. The renal disease was already present before treatment but was masked by the hyperdynamic state of the hyperthyroidism.
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