Abnormality in pku

Jacklynn
My baby boy got his first pku the day after he was born in the hospital and we had to make an appointment for the second one. Well today I was informed he had a slight abnormality in his thyroid... 
Seriously broke my heart, the doctor said he isn't concerned about it and Monday we go in to have it done again, anyone ever have a false positive? Could it have been possible that since he was roughly 3weeks early it affected the test and now he's alright? I just keep looking at my son thinking somethings wrong and I can't help him :(