Poor concentration is one symptom of underactive thyroid.
Are you being treated for it, or have you been told you are borderline? once bloodtests are done and the correct thyroxine dosage is taken your concentration level should return to normal.
I have been on thyroxin for 15 years due to the same condition but it has never affected my concentration whilst driving or any other time, maybe it affects people in different ways, i am clumsy, but I take my meds regularly and so i can't blame that.