Effect on Renal Function of Restoration of Euthyroidism in Hyperthyroid Cats with Iatrogenic Hypothyroidism

BACKGROUND: Iatrogenic hypothyroidism is associated with an increased incidence of azotemia after treatment of hyperthyroidism, and decreased survival time in azotemic hyperthyroid cats. HYPOTHESIS: Restoration of euthyroidism will decrease plasma creatinine concentrations. ANIMALS: Nineteen client‐owned, methimazole‐ or carbimazole‐treated, hyperthyroid cats with documented iatrogenic hypothyroidism (based on subnormal plasma total thyroxine concentrations [TT4] and increased plasma thyroid‐stimulating hormone concentrations). METHODS: Prospective interventional study. Doses of antithyroid medication were reduced until euthyroidism was restored (TT4 10–40 nmol/L). Plasma creatinine concentration and selected other clinicopathologic variables were evaluated before and after restoration of euthyroidism and compared by nonparametric statistics. Data are presented as median [25th, 75th percentile]. RESULTS: Restoration of euthyroidism was associated with a significant decrease in plasma creatinine concentrations (2.61 [1.90, 3.26] mg/dL versus 2.07 [1.42, 2.82] mg/dL; P < .001) and body weight (4.03 [3.59, 4.53] kg versus 3.89 [3.34, 4.18] kg; P = .019), and a significant increase in packed cell volume (30 [28, 39]% versus 34 [29, 39]%; P = .038), heart rate (174 [163, 201] bpm versus 190 [164, 202] bpm; P = .009), and plasma alkaline phosphatase activity (26.6 [17.0, 33.0] IU/L versus 38.0 [23.5, 46.5] IU/L; P < .001). CONCLUSIONS AND CLINICAL IMPORTANCE: Restoration of euthyroidism in medically treated hyperthyroid cats with iatrogenic hypothyroidism causes a reduction in plasma creatinine concentrations, and thus might improve renal function; however, this could be influenced by concurrent changes in body weight.