After 8 yrs.and gradually reducing Block and Replace, I recently stopped Carbimazole but remain on Levothyroxine 75/100mcg. alternate days. Recent Thyroid panel shows TSH 0.09 / fT4 60% thro range / fT3 40%. You say this swing to Hypo is common and may be transient. I presume I have Blocking TRAb dominating. What I don't understand is if this is the case would my TSH not be raised. I would like to try and understand and would be grateful if you could explain the mechanism behind it.

Hi,
About 20% of patients who move into remission using meds or naturally have a 20% of moving into hypotjyroidism. You're right in that the cause is blocking TSH receptor antibodies. All TSH receptor antibodies are recognized by the pituitary as if they were TSH. So the pituitary thinks you have enough TSH and stops secreting TSH normally. This causes a falsely lowered TSH result. I have the same problem with my TSH staying at <.07 even if I'm hypothyroid. That's why we say TSH is misleading in Graves' disease. You want to be closer to the high end of the range for FT4 and midway between the middle and high ends of the range for FT3. Best, Elaine