What does normal tsh, but high free t4 mean?

Recheck the labs: Normally, if the ft4 is high, either due to hyperthyroidism or taking too much synthroid, (thyroxine) your TSH is low. Mistakes in labs can occur, so first have them rechecked. Rarely, you can have a pituitary tumor that makes TSH and causes hyperthyroidism, giving you both increased TSH and ft4. But this is rare, and more likely there is a simple explanation.