Liz 1967 asked if your hormone replacement medication changed.
Are you taking a brand name of thyroxine? If you are, then you know that your medicine is made by the same manufacturer and the potency should remain the same. If you are taking a generic then you have to check when you refill that it is being made by the same manufacturer. There is suppose to be a 3 letter code on the label that identifies the manufacturer. The potency can vary for different brands as well generics.
I hope you find out the mysterious reason for your hypothyroidism soon.