Harley L.S., Peterson M.E., Langston C.E., et al.
Conference Proceedings, (2011). American College of Veterinary Internal Medicine:
Although it is well established that concurrent chronic kidney disease (CKD) develops in about 30% of hyperthyroid cats, no one has reported the use of the IRIS staging system for CKD before and after treatment of these hyperthyroid cats. The purpose of this study was to compare the effects of treatment in hyperthyroid cats with known stage 1 and 2 CKD in order to determine the effects of restoring euthyroidism or inducing hypothyroidism has on the IRIS stage in these cats. We evaluated 36 hyperthyroid cats (median age, 14 years) in this study. One day prior to treatment, serum T4 concentration, serum chemistry analysis, complete urinalysis, and urine protein-to-creatinine ratio (UPC) were measured. All cats were again evaluated with the same parameters again 3 months after treatment with 131I. Prior to treatment, 26 (72%) of the 36 cats had no evidence of azotemia (serum creatinine o 1.6 mg/dl), whereas 10 cats (28%) had stage 2 CKD (serum creatinine, 1.6–2.8 mg/dl). In the 36 cats, IRIS staging revealed proteinuria in 33 cats (92%), 21 with borderline proteinuria (UPC, 0.2–0.4) and 12 with overt proteinuria (UPC 4 0.4). Hyperthyroidism was cured in all 36 cats (median post-T4, 1.3 mg/dl). All cats had a good response to treatment; there were no signs of CKD except for polyuria and polydipsia in some cats. A significant (P o 0.001) increase in median values for both serum urea nitrogen (26 mg/dl to 31 mg/dl)) and creatinine (1.1 to 1.7mg/dl) occurred after treatment. Nine of the 26 cats (34.6%) classified as nonazotemic or IRIS stage 1 prior to 131I progressed to stage 2 CKD after 131I. All 10 cats with stage 2 CKD before treatment remained azotemic after 131I, with 5 cats remaining in stage 2 CKD, and 4 cats progressing to stage 3 CKD (serum creatinine, 3.1–3.7 mg/dl). There was a significant inverse relation- ship (P 5 0.002) between pretreatment urine specific gravity (USG) and post-treatment serum creatinine in the 36 cats. Of the 19 cats with post-treatment serum creatinine values 4 1.5 mg/dl (stage 2 to 3 CKD), 15 (79%) had pretreatment USG of o 1.040. In contrast, in the 17 cats with post-treatment serum creatinine values o 1.5 mg/dl, only 3 (18%) had pretreatment USG of o 1.040. A significant (P < 0.001) decrease in median UPC from 0.3 to 0.1 occurred after treatment, but there was no relationship between degree of proteinuria and IRIS stage in these cats. Two cats developed iatrogenic hypothyroidism after 131I, diagnosed by finding low serum T4 and high cTSH concentrations. Both hypothyroid cats had pro- gressed from stage 1 before treatment to stage 2 and 3 CKD, respectively, after 131I; after thyroxine replacement, serum creatinine decreased to near pretreatment concentrations in both cats. Con- clusions: 1) IRIS stage 2 CKD is common in untreated hyperthyroid cats. 2) Progression to next higher IRIS stage is common after treatment, but most cats with remain relatively asymptomatic for CKD. 3) USG may be helpful in predicting which cat’s IRIS stage will progress after 131I. 4) Iatrogenic hypothyroidism worsens azotemia, an effect that appears reversible with replacement therapy.