Re: thyroid cancer?
If it is thyroid related it would be serious insofar as you may need to be on a tiny hormone daily to replace what your thyroid may not be producing (which is usually the case).
Your symptoms do not necessarily sound like they are thyroid related. They sound sinus related to me. I have sinus drainage that comes and goes. Your symptoms sound like what I experience when my sinuses are acting up.
Do you live with your parents or a relative? If so, you need to talk to them and see about making a doctor's appointment so you can be properly diagnosed.
Posters on this board are very knowledgable because most are diagnosed with thyroid conditions and have done their homework but, "we" are not professionals here. I strongly advise you to get diagnosed FIRST. Then use this forum as a support mechanism.
We don't diagnose here, we support.
Good luck and let us know how it goes.