KMy 13 Year Old Son's TSH Was Discovered 2 Years

KMy 13 year old son's TSH was discovered 2 years ago. It was 12.4 at that time. It went up to 17. He was then started on levothyroxine 0.25 Mcg. He had taken this for a year. Now his thyroid was 21.0 about 4 months ago. He continued on his meds at at same dose. We just did a TSH draw again and now his TSH is 23!!!
What could possibly be going on??

Thanks!!!
Marneson