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

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.
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.