What can cause high tsh level after several years of synthroid (thyroxine) for post ablative (graves) hypothroidism?

Usually low Thyroid : Post ablative hypothyroidism is a condition where the thyroid gland is not functioning well and thus the patient needs more thyroid hormone. A "high tsh" actually mean there is not enough thyroid in the blood, thus an increase in dosage of the synthroid (thyroxine) is needed. Go see your family doctor and he/she will make adjustments and recheck the TSH level. This is very sensitive to thyroid in the body.

Several: 1. Missing doses 2. Taking with other meds or food 3. Taking generic levothyroxine but frequently missed reason is: you gained weight, will need more synthroid.