My TSH and FT-4 has done this with every 10 week testing for 11 years. Can't figure it out. I am treated Hyper/Graves' with some thyroid function left.
Maybe your thyroid is trying to grow back or antibodies are attacking what tissues is left from surgery.
Next test with TSH and FT-4 have them do a FT-3, to see how they relate to each other and if there might be another thyroid issue going on.
Good Luck!
The thyroid levels take time to stabalize. I know I went through many years of trying to stabalize. My doc recommended only synthroid because he said the levels were more stable. Also, he recommended asking the pharmacy what date it was made and sent, as it loses potency over time. It should be fresh from the pharmacy - and you can ask to find out. I think, too, the stressors we deal with in life everyday, depending on the level of stress, can increase or decrease the amount we expend. That's just a thought. It's tough to focus on reducing stress when we're so stressed out from the symptoms of the disease. You must have felt awful and probably still do. Just keep faith and get that lab work.