How do I know whether thyroid cancer is life threatening?

It's complicated. Some thyroid cancers are found "incidentally" during surgery in when cancer is not suspected. When these are small and completely within the thyroid they are cured by surgery and have a 0% chance of coming back. Some types, such as anaplastic, have a nearly 100% fatality rate. Most are in between, in the 2-20% fatality range. Your doctor can give you a better answer for your situation.
It is cancer! By definition thyroid cancer is life threatening... There are three main types of thyroid cancer 1) papillary 2) follicular 3) medullary. Anaplastic thyroid carcinoma is an uncommon but rapidly fatal thyroid cancer. Cancer is stages by ages 1) age at onset (younger is better) 2) gender (female is better) 3) extent of disease 4) size. Thyroid cancer and it's treatment is complex.