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 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.

Urine concentrating ability in cats with hyperthyroidism: Influence of radioiodine treatment, masked azotemia, and iatrogenic hypothyroidism

“…Although the goal of 131 I treatment is to cure hyperthyroidism and restore euthyroidism, most current dosing protocols fail to achieve this goal, with 20% to 50% of 131 I‐treated cats developing iatrogenic hypothyroidism after treatment 9,32,36,38,58,59 . Determination of thyroid status is of vital importance when assessing these 131 I‐treated cats for CKD, because cats with iatrogenic hypothyroidism have a higher prevalence of azotemia than euthyroid cats, 2,14,25,32,36 as did the hypothyroid cats in this study (2.2‐fold higher than our euthyroid cats; Figure 4B; Table S5). The azotemia that develops in cats with iatrogenic hypothyroidism, however, can sometimes be improved or even reversed with levothyroxine supplementation 25,60 .…”

“…[1][2][3] This is of clinical importance, since hyperthyroidism is a reported risk factor for chronic kidney disease (CKD), [4][5][6] in which a USG <1.035 is expected. 7,8 Furthermore, hyperthyroidism can complicate (mask) the diagnosis of azotemic CKD by increasing renal blood flow and glomerular filtration rate (GFR) [9][10][11][12] and decreasing body muscle mass, 13,14 thereby lowering serum creatinine concentration to within its reference interval. Consequently, many hyperthyroid cats with concurrent CKD only develop azotemia after successful treatment, when hyperthyroidism no longer maintains high renal blood flow and GFR.…”

Serum metabolome analysis in hyperthyroid cats before and after radioactive iodine therapy

“…Decreased creatine phosphate in hyperthyroid cats and increased creatine in hyperthyroid humans may reflect species differences or could imply altered conversion. Lower creatine phosphate (phosphocreatine) may reflect lower muscle masses often seen in hyperthyroid cats [ 11 , 12 ]. As creatine phosphate serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle, myocardium, and the brain, reduced serum levels also reflect changes in energy metabolism at these sites [ 13 , 14 ].…”

Serum metabolome analysis in hyperthyroid cats before and after radioactive iodine therapy

“…Lower creatine phosphate (phosphocreatine) may re ect lower muscle masses often seen in hyperthyroid cats. 34,35 As creatine phosphate serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle, myocardium, and the brain, reduced serum levels also re ect changes in energy metabolism at these sites. 36,37 In this study, post-I-131 treatment creatine phosphate levels improved but were still signi cantly reduced compared to controls, further indicating persistent negative energy balance in previously hyperthyroid cats.…”