It depends on which stage of hypothyroidism you're in and how long the process has been going on. Long term effects include:
Slowing down of your metabolic processes
Weight gain
Fluid retention—swelling in the face, ankles and feet
Significant cognitive deterioration
Neuropathy symptoms
Increased cholesterol
Issues with fertility
Depression
One of the most feared complications is Myxedema Coma. This is a comatose state that happens when an individual with severe hypothyroidism has stopped taking their thyroid medication. It requires ICU admission and has a very high mortality rate. It can be easy to think that you don’t need your thyroid medication because for the first few days to two weeks you will not notice any significant change. It takes approximately six to twelve weeks for your dose to equilibrate in your body.