My 6 year old DS1 has 2 white hairs on his head too, I first noticed them several years ago. It's not a "lighter in the sun" thing for him either (my kids are 1/4 Korean), I'm not sure what causes them.
SurbubanHippie - how did you find out yours were related to thyroid function? What supplement are you looking at getting? (I'm thinking for me, not for DS1, of course.)