If someone has been off of thyroid medication for an underactive thyroid how long can that person live without the medication?

It depends: Thyroid hormone is an essential hormone for proper function of many organ systems in the body. People with no thyroid function (such as people who have had their thyroid removed) will develop significant hypothyroid symptoms within a few weeks of stopping medication. People with mildly low underlying thyroid function can go much longer. Why would you want to stop the medication it?