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.

Keywords: 131I; cachexia; feline; muscle condition; radioactive iodine; sarcopenia; thyroid gland; ultrasound.

FIGURE 1

Flowchart for enrollment of hyperthyroid cats, into study 1 (cross‐sectional study of untreated cats) and study 2 (before‐after treatment)

FIGURE 2

Boxplots of the epaxial muscle height (EMH) in 120 untreated hyperthyroid cats and 48 clinically normal euthyroid cats. Boxes represent the interquartile range from the 25th to 75th percentile. The horizontal bar in each box represents the median value. The whiskers indicate the range of data values unless outliers are present, in which case the whiskers extend to a maximum of 1.5 times the interquartile range. Such outlying data points are represented by open circles. The shaded area indicates the reference interval

FIGURE 3

Boxplots of the length of the 4th thoracic vertebrae in 120 untreated hyperthyroid cats and 48 clinically normal euthyroid cats. See Figure 2 for key

FIGURE 4

Boxplots of the forelimb circumference in 120 untreated hyperthyroid cats and 48 clinically normal euthyroid cats. See Figure 2 for key

FIGURE 5

Boxplots of the vertebral epaxial muscle score (VEMS) in 120 untreated hyperthyroid cats and 48 clinically normal euthyroid cats. See Figure 2 for key

FIGURE 6

Boxplots of the forelimb epaxial muscle score (FLEMS) in 120 untreated hyperthyroid cats and 48 clinically normal euthyroid cats. See Figure 2 for key

FIGURE 7

Boxplots of the epaxial muscle height (EMH) in 75 hyperthyroid cats evaluated before and after treatment with radioiodine. See Figure 2 for key

FIGURE 8

Boxplots of the vertebral epaxial muscle score (VEMS) in 75 hyperthyroid cats evaluated before and after treatment with radioiodine. See Figure 2 for key

FIGURE 9

Boxplots of the forelimb circumference in 45 hyperthyroid cats evaluated before and after treatment with radioiodine. See Figure 2 for key

FIGURE 10

Boxplots of the forelimb epaxial muscle score (FLEMS) in 48 hyperthyroid cats evaluated before and after treatment with radioiodine. See Figure 2 for key