Do you have the numbers for her recent blood work? Was a urinalysis also done? Depending on what the T4 value is, she could still be hyperT even if the value is within the lab's normal range. Another test to help with diagnosis is a Free T4 done at the same time that the T4 is tested: http://www.vet.uga.edu/vpp/clerk/stortz/index.php
Diagnosis of hyperthyroidism becomes more difficult if the T4 and T3 values are within the reference interval. This occurs in approximately 10% of cats with hyperthyroidism.7
Another test for the confirmation of feline hyperthyroidism is the measurement of free T4 concentration by dialysis. The free T4 concentration is that circulating T4 which is unbound to carrier proteins. Free T4 is metabolically active; its measurement could give a more accurate assessment of thyroid function.6 Free T4 is also less likely to be affected by nonthyroidal disease or drug administration than is total T4. The free T4 test is significantly more sensitive in detecting hyperthyroidism in mildly hyperthyroid cats than either total T4 or T3 measurements. However the free T4 test occasionally has a false positive test result, so hyperthyroidism should not be diagnosed solely on the basis of free T4 determination.11 Other clinical signs such as a palpable thyroid nodule, weight loss, etc. (Table 1) also should be present for a definitive diagnosis of hyperthyroidism.