I have exactly the same problem. Actually I had 2 partial thyroidectomys. The first one 33 years ago and the last one in May 2007. After the first one a multinodular goiter grew back on the remaining right side. This took several years. After the one in May a mass grew back on the left side after a few months. I went to see the endo a few weeks ago and he said the remaining thyroid is overcompensating for the part that isn't there. All of my TSH levels are on the low side of normal but my microsomal antibodies are high. He feels the remaining thyroid is getting ready to fail and put me on synthroid. None of mine so far was cancer but I'm at high risk because I had xray treatments to the head and neck as a child. Go to the endo and see what he says. Mine wants to monitor it every couple of months. I'm scheduled for an ultrasound the end of March. Good luck and believe me I know exactly what you are going through.
I didn't have any abnormal labs or any thyroid disease. A thyroid nodule was found on a completely unrelated CT scan. I had the right lobe removed after the ultrasound showed a small, complex nodule. It was papillary carcinoma. I had the other 1/2 out the following year after 3 of my sisters also were found to have papillary carcinoma, the other precancer, and my daughter precancer.
Nodules can grow in any thyroid tissue at any time. 90-95% of them are benign. I have read that 30% of 30 year olds have thyroid nodules, 40% of 40 year olds, etc.
Here's some info that may help:
http://www.endocrineweb.com/thyroid.html
There are a lot of us on here who have nodules and goiters - maybe you'll get more response tomorrow.
What did the ultrasound of the new nodule/mass show??