Peterson M.E., Castellano C. and Rishniw M.
Conference Proceedings, (2016). American College of Veterinary Internal Medicine, Denver:
Hyperthyroidism is a catabolic state associated with increased energy expenditure, increased lipolysis, and increased protein turnover. In human hyperthyroid patients, weight loss is predominantly due to the decrease in lean body mass (primarily muscle) with the loss of fat being less important (at least initially). After treatment, recovery of lost muscle mass generally occurs before fat deposits are completely restored. In hyperthyroid cats, the contribution of muscle wasting (sarcopenia) to the overall loss of body weight is not known. However, many cats are currently being diagnosed at an early or mild stage of hyperthyroid disease and display minimal weight loss or other clinical signs. If these cats are similar to humans, loss of muscle mass might still be expected, but again, this has not been reported.
Our aim in this study was to investigate the body weight, body condition score (BCS), and muscle condition score (MCS) of a large population of hyperthyroid cats, both before and after successful treatment with radioiodine. We studied 436 hyperthyroid cats, of which 60 had a follow-up examination 3–12 months after successful treatment. For inclusion, all cats had a detailed history (including diet fed) and complete physical examination. Diagnosis was confirmed by use of a complete serum thyroid panel (T4, T3, fT4, TSH) and results of thyroid scintigraphy. Body weight, BCS (5-point scale) and MCS (4 point scale) were measured (J Small Anim Pract. 2011; 52:385) in all cats by a single investigator (MEP).
Before treatment, body weight in the 436 cats ranged from 1.9 to 8.8 kg (median, 4.36 kg); this hyperthyroid weight was significantly less (p <0.0001) than the premorbid weight (5.45 kg) recorded 1–2 years prior to diagnosis. The median pre-treatment BSC was ideal (3/5); of the 436 cats, 72 (16.5%) were overweight, 210 (48.2%) were ideal, and 154 (35.3%) were underweight. The median pre-treatment MCS was consistent with mild muscle loss (2/3); of the 436 cats, 94 (21.6%) had a normal MCS, whereas mild, moderate, and severe muscle loss was recorded in 173 (39.7%), 128 (29.4%), and 41 (9.4%), respectively.
Weak but significant (p < 0.05) correlations were found between serum T4 and both BCS (r = 0.16) and MCS (r = 0.15). When the 436 cats were subdivided into 3 groups based on the magnitude of the serum T4 value (< 7 μg/dl; 7–
12 μg/dl; > 12 μg/dl), the prevalence of both low BSC and moderate to severe muscle wasting increased with worsened severity of disease (p < 0.05). Significant (p < 0.0001) correlations were also found between age and both BCS (r = 0.25) and MCS (r = 0.23). When the 436 cats were subdivided into 3 groups based on their life stage (mature, senior, or geriatric), prevalence of low BSC and moderate to severe muscle wasting was significantly higher (p < 0.005) in geriatric cats than the younger groups. There was no difference in median serum T4 concentrations between cats of these 3 age groups. Finally, no relationship between the diet fed (dry, canned, or both) and BCS or MSC could be identified.
Median body weight, BCS, and MCS all increased significantly (p < 0.001) in the 60 cats re-evaluated after treatment. Of these, 21 (35%) were originally underweight: after treatment, 18 had an increase in BCS, 3 were stable, and 6 remained underweight. Of the 60 cats, 47 (78.3%) originally had low MSCs; after treatment, MCSs normalized in 16, improved but remained low in 20, and showed no change in 11.
In conclusion, most hyperthyroid cats lose body weight but maintain an ideal body condition or are overweight, with only about a third being underweight. However, loss of muscle mass (sarcopenia) is very common (over 75%). Severity of hyperthyroidism and older age both contribute to lowered BCSs and MCSs. Successful treatment leads to weight gain and increase of BSC in most cats, many times into the overweight range. Improvement in low MCSs are seen in most cats after treatment, but many cats fail to regain normal MCSs. Further studies are needed to investigate the role of nutrition (especially protein intake) in the pathogenesis of the low MCS observed in cats with hyperthyroidism.