Feline hyperthyroidism (HT) is a common endocrine disorder worldwide, but clinical and laboratory features might vary geographically. The aim of this retrospective study was to evaluate feline HT in Japan, and compare results to those of previous study for feline HT. We evaluated 48 feline HT cases clinical and laboratory features. Surprisingly, the youngest patient was 32 months of age (2 year 9 months). There was no significant difference among the study subjects in sex, but frequency of spayed/castrated cats was high (85.4%). Median age was 186 months (32-272 months). 91.3% (n=42) of subjects were over 10 years of age, and 8.7% (n=4) were under 10 years of age. Clinical symptoms included vomiting, 56.3% (n=27); diarrhea, 2.1% (n=1); hyperactivity, 12.5% (n=6); emaciation, 41.7% (n=20); polyuria and polydipsia, 22.9% (n=11); chronic weight loss, 60.4% (n=29); and palpated enlarged thyroid, 2.1% (n=1). Concurrent findings included chronic kidney disease, 20.8% (n=10); congestive heart failure, 20.8% (n=10); tachycardia (over 240 beats/min), 18.8% (n=9); gallop rhythm, 31.3% (n=15); neurological disorders such as hind-limb paralysis, 14.6% (n=7); cystitis, 8.7% (n=4); gingivitis, 4.2% (n=2); diabetes mellitus, 4.2% (n=2); and arterial thromboembolism, 6.3% (n=3). In addition, laboratory features (complete blood counts and biochemistry) differed from those of previous reports in certain respects. Our results show that it might be important for practitioners to comprehend epidemiologic differences regarding feline HT worldwide.