I was diagnosed with thyroiditis, but told there was not any treatment?

Most of the time. But all of them will not will not become hypothtroid , as it depends on the cause, like in hashimoto's, riedel's , due to radiation, due to drugs , how much functioning thyroid tissue is damaged , but if it is due to infections ( mostly virus ) mot of the thyroid will recover and will be euthyroid ( reverts to normal ).