Twin - I would say it is very possible.

My cat, Figaroe, showed signs of hyperthyroidism but the "normal" test they requested came back with normal results. I took Figaroe to see a specialist in Ohio who checked him out and said he is hyperthyroid. I gave her copies of his panels and pointed out all were normal. She sent me home with the instructions for a specific test (wish I could remember what it was). The blood was draw and two days later his results came back. His values were so high they did not register. He underwent the radioactive iodine therapy and is now hypothyroid but hey it is controlled with a daily dose of medication.

If you are not happy with the doctor or suspect that your results are not thorough seek a second or third opinion. Educate yourself about the disease and don't be afraid to ask for specific tests. Good luck and keep us posted.