EN15 - Relationship Between Urine Concentration and Development of Azotemia After Treatment of Hyperthyroid Cats with Radioiodine

Abstract: Background- Many clinicians believe that hyperthyroidism can cause polyuria and sub-optimally concentrated urine in cats. Prevalence of dilute vs. concentrated urine concentration in hyperthyroid cats, and if this changes after treatment, are unknown.
Objectives- To determine proportion of hyperthyroid cats with serum creatinine (SCr) < 2.0 mg/dL and USG < 1.035 prior to treatment. To determine if urine concentration changes after euthyroidism is restored, and whether an initial USG < 1.035 predicts post-treatment azotemia (SCr >2.0 mg/dL).
Animals- 614 hyperthyroid cats.
Methods- Prospective, before-after study. Cats had serum T4, T3 and creatinine concentrations and USG measured before and 6 months after successful treatment with radioiodine. Cats with pretreatment serum creatinine >2.0 mg/dL or treatment failure were excluded.
Results- 296/614 (48.2%) hyperthyroid cats had USG < 1.035, and 318/614 (51.8%) had USG ≥1.035 before treatment. After treating cats with initial USG < 1.035, 257/296 (86.8%) remained < 1.035, but 39/296 (13.2%) became concentrated (Figure). After treating cats with USG ≥1.035, most still had USG ≥1.035, but 63/318 (20.4%) became USG < 1.035. Fewer cats with pre-treatment USG ≥1.035 (19/318; 6.0%) developed post-treatment azotemia than cats with pre-treatment USG < 1.035 (121/296; 40.9%; P< 0.001, Chi-squared). USG < 1.035 had a high negative predictive value (94%) for post-treatment azotemia, but poor positive predictive value (41%).
Conclusions/clinical importance- Hyperthyroid cats have equal prevalence of UGS ≥1.035 vs. < 1.035, which tends not to change after radioiodine treatment. Of cats with USG < 1.035, nearly half developed SCr >2.0 mg/dL after treatment, whereas cats with USG ≥1.035 had a low probability of developing SCr >2.0 mg/L.