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.

Objectives Although radioiodine (131I) is the treatment of choice for feline hyperthyroidism, 131I-dosing protocols commonly induce iatrogenic hypothyroidism and expose azotaemia. A recently reported patient-specific 131I dosing algorithm minimised the risk of 131I-induced hypothyroidism and azotaemia, while maintaining high cure rates. The aim of the study was to report results of 131I treatment in a European population of hyperthyroid cats using this patient-specific dosing algorithm. Methods This prospective case series (before-and-after study) evaluated 165 hyperthyroid cats referred for 131I treatment. All cats had serum concentrations of thyroxine (T4), triiodothyronine (T3) and thyroid-stimulating hormone (TSH) measured (off methimazole ⩾1 week). Thyroid volume and percentage uptake of 99mTc-pertechnetate (TcTU) were determined using thyroid scintigraphy. An initial 131I dose was calculated by averaging dose scores for T4/T3 concentrations, thyroid volume and TcTU; 70% of that composite dose was then administered. Twenty-four hours later, percentage 131I uptake was measured, and additional 131I administered as needed to deliver an adequate radiation dose to the thyroid tumour(s). Serum concentrations of T4, TSH and creatinine were determined 6–12 months later. Results Median calculated 131I dose was 2.15 mCi (range 1.2–7.5), with only 51 (30.9%) receiving ⩾2.5 mCi. Of 165 cats, 124 (75.2%) became euthyroid, seven (4.2%) became overtly hypothyroid, 27 (16.4%) became subclinically hypothyroid and seven (4.2%) remained hyperthyroid. A higher proportion of overtly (85.7%) and subclinically (26.9%) hypothyroid cats developed azotaemia than euthyroid cats (13.6%; P = 0.0002). Hypothyroid cats were older ( P = 0.016) and more likely to have detectable TSH concentrations ( P = 0.025) and symmetrical bilateral distribution of 99mTc-pertechnetate uptake ( P = 0.0002), whereas persistently hyperthyroid cats had higher severity scores ( P = 0.012). Conclusions and relevance Our results confirm that 131I dosing with this new algorithm results in high cure rates, with a lowered prevalence of 131I-induced overt hypothyroidism and azotaemia. Age, serum TSH concentrations, bilateral, symmetrical uptake and severity score help predict outcome.