I had a consistently enlarged lymph node in my neck for almost 2 years that turned out to be thyroid cancer.
My doctors were not concerned, but I was, and continued to push for follow up and tests. All along I was told nothing was wrong. Once we finally found nodules on the thyroid, I was consistently told there was a very low chance it was cancer, etc.
It took 2 years to get a diagnosis, which did not occur until I went in for thyroid surgery--literally while I was on the operating table. My TSH levels were normal all the way through. Whatever nodules were on my thyroid did not get detected for about 1.5 years--my guess is they were too small to be felt on exam, even though the cancer was in there.
Initially, I was told it (the lymph node) could not be biopsied due to its location (along the corotid artery). Later, another doctor said it could be--by using an ultrasound guided biopsy--but we were close to my (thyroid) surgery date so I opted not to.
An ultrasound can also be done on a persistent, enlarged lymph node--I asked the radiologist to "look" at it while I was in there for an ultrasound-guided biopsy of my thyroid. We discovered calcifications in the lymph node, which FINALLY lit a fire under my ENT to schedule surgery.