The fluctuations have not much to do with your changed thyroid
dose. The doctor of yours has done a right thing by bringing your thyroid medication dose down as you are beyond 70 years.
The sugars are not getting controlled by the thyroid medications. Due to your old age the tablet Glipizide
is getting metabolised slowly and hence the sugars fluctuate. Please discuss with your physician about use of alternative oral medications for diabetes
for smooth control.
One more possibility can be that your glucometer can be at fault and hence the readings are so. Please make sure the meter is calibrated well.
I hope you got your answer. Take care.