Blood sugar problems can sometimes hinder the proper action thyroid hormone.
On of insulin's roles is to act on the membranes that surrounded each of our cells. Insulin enables the cell membranes to allow blood sugar to enter the cells.
If blood sugar is rising too high at times then this may mean one of two things:
1) Insulin is too low and therefore this stops enough glucose from entering the cells - and the blood levels of glucose are high at times. Diabetes is the extreme case of this but pre-diabetes (early beginnings of diabetes can also do this).