Levothyroxine is the most commonly prescribed thyroid medication on the market and many people have no idea what kind of side effects it can cause. Here's the list that every thyroid patient should know about if they are taking it: 1. Weight gain 2. Increased cancer risk 3. Hair loss 4. Dementia risk 5. Bone density concerns The good news is that up to 30% of all thyroid patients reading this may be able to get off their thyroid medication, so these are not necessarily side effects that you have