The measurement of glycosylated hemoglobin, also known as HbA1C, is often described as a "diabetic truth detector." A measurement of the percentage of red blood cells that are coated with sugar, HbA1C goes up when blood sugar levels generally run high and goes down when blood sugar levels generally run low. Every high HbA1C reading, however, is not caused by high blood sugar levels.
When people have slow thyroid function, or hypothyroidism, the body doesn't recycle old red blood cells o make new red blood cells as quickly. The longer red blood cells stay in circulation, the more sugar they attract. In the worst cases of hypothyroidism, some people have been misdiagnosed as prediabetic because their HbA1C's levels were unexpectedly high.
More often than not a high HbA1C level really does mean that blood sugar levels have been going too high. At least once every five years, however, every diabetic should have tests for TSH, T4, and T3 to make sure that thyroid function is normal.