Thyroid cysts are extremely common and most are benign. It is possible that the swollen lymph node could be connected but it could also be complete coincidence. Apparently, many people have cysts on their thyroids that are only discovered when they get an ultrasound or other test for another issue on their neck.

Only about 5% of thyroid cysts are cancerous, as far as I know.

And even in the worst case scenario, if it is cancer, thyroid cancer is one of the least severe forms of cancer (or at least most forms are and the serious form is very rare - like 1% of thyroid cancers or something - and mostly affects older people). The most common form, papillary thyroid cancer has something like a 99% survival rate.

And they don't usually use chemotherapy or radiotherapy to treat it. It's generally a matter of removing the thyroid and then, if they feel it necessary, doing radioiodine treatment. Neither of these things are that serious. I had papillary thyroid cancer and was back at work a month after my operation, if that tells you anything. And the radioiodine treatment is really just a nuisance. It's basically just a matter of swallowing a capsule, but you have to be in isolation after it because you are radioactive. It's just for the protection of others though, not because you are at any risk.

Best of luck. I hope everything will be found to be completely benign and harmless.