I think it would be fairly uncommon for thyroid cancer to spread to the breast or armpits. I had thyroid cancer two and a half years ago and did a lot of googling and so on and have never come across any references to such things. I know I am not an expert, but generally google tends to give more worrying suggestions than the reality. 

The most common forms of thyroid cancer rarely spread outside the neck (I think for papillary thyroid cancer, it's 1%-4% of case that have spread and so far, you don't even know if you have thyroid cancer, the odds are still that you don't) and even when it does, it's most commonly to the lungs. Spread to the bones and some other places is rare but also happens. 

I have no idea whether spread such as you are thinking of could happen; like I said, I am no expert, but it certainly isn't something I have ever heard of happening.

Thyroid cancer is usually slow growing and rarely causes serious issues. Thyroid cysts, both cancerous and benign can cause difficulty breathing; I know that because even before my results came back, the consultant said I would likely need surgery anyway, as the cyst could cause difficulty breathing if it were to grow any further. That was when he was still implying it was likely benign.

I will say I had no symptoms whatsoever with thyroid cancer, apart from a lump in my neck. This, I think, is common.

Hope you get an answer to what is causing your problems and that it turns out to be nothing too serious.

I will add that the most common form of thyroid cancer has something like a 77% survival rate even if it has spread outside the neck, so even in the absolute worst case scenario, the odds are very much in your favour, but so far, the chances are that it's not even cancerous and if it is, it is pretty unlikely to have spread beyond the thyroid and lymph nodes.

That is very reassuring thank you! Hopefully the surgery will go well and at least I'll be able to breathe easier! I have been so very unwell for the past few months it is hard not to worry! I am going to  get scans done to try and get to the bottom off things! My mum, aunt and sister all passed away from different forms of cancer, which weren't picked up on until it was too late and my cousin has Leukaemia. So I guess I am being super cautious!! 

You have been through a lot. Not surprising you'd be really worried after all that. Buy thyroid cancer is rarely like we mostly imagine cancer. It's usually just a matter of having your thyroid removed.