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.