Diagnosing hyperthyroidism (too much thyroid hormone) is much easier than hypothyroidism (too little thyroid hormone).  A blood sample is collected and analyzed for the total amount of T4.  In general the normal range of T4 for a cat is 0.5 to 4.7 micrograms/deciliter, although this might vary slightly depending upon the lab.  A T4 greater than 4.7 is indicative of hyperthyroidism.  Cats with values in the upper end of normal might require additional testing to detect hyperthyroidism.     

Because hyperthyroidism is often associated with other medical problems, I always include a complete blood count (CBC), chemistries and a urinalysis with the total T4.  We also check blood pressure as many older cats suffer from hypertension.  Lastly, the ocular pressure is measured.  

The results of these tests will determine the treatment options available for the cat.  Because hyperthyroidism may mask kidney disease, it is extremely important to monitor kidney function.  Before pursuing a permanent treatment for the excess T4, I like to perform a therapeutic trial of methimazole, a medicine that binds thyroid hormone.  When the total T4 is within normal limits, I recheck blood and urine to make sure the cat's kidneys are functioning well.