I was trying to add to our discussion but my brain is not technical tonight. I had a reoccurence happen 30 years after the first. The treatment was SO much easier with the exception of having to go without thyroid for 3 months. That was really hard!
The way it has been explained to me is that the endocrine system responds to stress by triggering a call through your body for the hormones produced by the adrenal glands and the thyroid (as well as others) those with thyroid cancer need to make sure that this call is not answered. Other cells will try to become the answering response by becoming thyroid and then become malignant. In my case it was a lymph node since it had spread before being found.
They don't normally test for T3 since most people are able to create T3 out of T4 and don't need it specifically. Every few years I do a specific test that tests for antibodies that gives a result to indicate whether we need to do further testing for cancer. This is an expensive one :-)
As you can probably tell I have lived without a thyroid for a very long time, If you ever want to talk or exchange emails let me know. You do need to have a specialist who knows about thyroid cancer. Another good resource is
http://www.thyca.org/ - Tracy