I Had Multi-nodular Toxic Goiter In 1995. It Was Treated

I had multi-nodular toxic goiter in 1995. It was treated with radioactive iodine ablation. After a few months, I started taking 0.125 thyroxine and have been ever since. My post ablation TSH levels were always normal or slightly less than normal for 20 years. Now I am 67 and all of a sudden my TSH is up to 12. How common is thyroid regeneration? I don't know if that is what is happening. My doctor doesn't seem to know. What is the recommended next step? More testing? Ultrasound? Increase of thyroxine dosage?