[header] How to diagnose high thyroid levels in a cat [title] Determine the age of your cat. [step] Elevated thyroid levels in cats is most common as cats age. In fact, most cats who develop hyperthyroidism are over the age of 13. 
Only 5% of cats with hyperthyroidism are less than 10 years old. If your cat is over 10, it is more likely that symptoms may indicate hyperthyroidism than in younger cats.