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Hello. I'm sorry to hear about your dog.
With metastasis present, there is not much you can do for these dogs. The absolute best way to treat would be to use radioactive iodine therapy to selectively kill the thyroid tissue in the body. This is only offered in a handful of places in each state, so availability is an issue. They are also radioactive during the treatment, so they have to stay in the hospital for a few days. With this treatment, though, we can have survival times of ~ 22 months even with metastatic cancer.
The other option is to use chemotherapy. This may only get you another few months, but it's an option.
The prognosis without any treatment is a survival time ~ 3 months after the diagnosis.
The last thing you could try is Methimazole. It is what we treat benign thyroid tumors in cats with. It may help alleviate some signs and improve quality of life. It's not used much in dogs, but it's an option.
I hope this helps, let me know if you have any other questions.
And also look into radioactive iodine. That gives you the best response and it's non-invasive.