Effect of radioiodine treatment on muscle mass in hyperthyroid cats.

Abstract

BACKGROUND: Approximately 75% of hyperthyroid cats lose muscle mass as accessed with a muscle condition scoring (MCS) system. After treatment, MCS improves as the cats regain muscle mass. OBJECTIVES: To quantify the degree of muscle loss in hyperthyroid cats using ultrasonography and evaluate changes in muscle mass after treatment. ANIMALS: Forty-eight clinically normal cats and 120 cats with untreated hyperthyroidism, 75 of which were reevaluated after radioiodine-131 therapy. METHODS: Prospective cross-sectional and before-after studies. All cats underwent ultrasonography and measurement of epaxial muscle height (EMH), with subsequent calculation of vertebral and forelimb epaxial muscle scores (VEMS and FLEMS). A subset of hyperthyroid cats underwent repeat muscle imaging 6&#xa0;months after treatment. RESULTS: Untreated hyperthyroid cats had a lower EMH than did clinically normal cats (median [25th-75th percentile], 0.98 [0.88-1.16] cm vs 1.34 [1.23-1.58] cm, P&#x2009;<&#x2009;.001). Seventy-seven (64.2%) untreated cats had subnormal EMH. Similarly, compared to normal cats, hyperthyroid cats had lower VEMS (0.93 [0.84-1.07] vs 1.27 [1.18-1.39], P&#x2009;<&#x2009;.001) and FLEMS (1.24 [1.10-1.35] vs 1.49 [1.39-1.63], P&#x2009;<&#x2009;.001). After treatment, EMH increased (1.03 [0.89-1.03] cm to 1.33 [1.17-1.41] cm, P&#x2009;<&#x2009;.001), with abnormally low EMH normalizing in 36/41 (88%). Both VEMS (0.94 [0.87-1.10] to 1.21 [1.10-1.31], P&#x2009;<&#x2009;.001) and FLEMS (1.31 [1.17-1.40] to 1.47 [1.38-1.66], P&#x2009;<&#x2009;.001) also increased after treatment. CONCLUSIONS AND CLINICAL IMPORTANCE: Almost two-thirds of hyperthyroid cats have abnormally low muscle mass when measured quantitatively by ultrasound. Successful treatment restores muscle mass in >85% of cats. EMH provided the best means of quantitating muscle mass in these cats.