Effect of radioiodine treatment on muscle mass in hyperthyroid cats

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 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 < .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 < .001) and FLEMS (1.24 [1.10‐1.35] vs 1.49 [1.39‐1.63], P < .001). After treatment, EMH increased (1.03 [0.89‐1.03] cm to 1.33 [1.17‐1.41] cm, P < .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 < .001) and FLEMS (1.31 [1.17‐1.40] to 1.47 [1.38‐1.66], P < .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.