Diagnosed pap thyroid ca august, Synthroid (thyroxine) 125 started, given iodine therapy. Tsh novem 1.8 now 6.1 on same synth dose. Causes?

Many possibilities. Many factors can affect thyroid hormone and TSH levels for people taking thyroid tablets. The most common is probably people forgetting pills. Forgetting as few as one pill every two weeks could lead to the kind of TSH increase you mentioned. Other causes could include weight gain, change in estrogen levels, taking thyroid pills with calcium or iron supplements, or with food. Work w/ your doc!