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The thyroid gland regulates energy metabolism, so both an overactive thyroid (called hyperthyroidism) and an under active thyroid (called hypothyroidism) can affect your diabetes control. If your thyroid medication is maintaining your thyroid function at a normal level, then there should be no effect on your diabetes control.

Hyperthyroidism:

Too much thyroid hormone causes more glucose to be produced from the liver, faster absorption of glucose through the intestines, and increased insulin resistance (resulting in the possibility of raising blood glucose). Furthermore, since people with diabetes have an increased risk of heart disease, and hyperthyroidism causes rapid heart rate, there is an increased risk of chest pain or other heart problems.

Hypothyroidism:

Not having enough thyroid hormone generally does not cause any significant changes in blood glucose control, although the slower metabolism can reduce the clearance of insulin from the bloodstream, sometimes resulting in a need for a reduced dose. More commonly though is that hypothyroidism is often associated with abnormal blood lipids (blood fat levels). Again, since many people with diabetes have heart disease and/or abnormal lipids, hypothyroidism can worsen these problems.

The above information is from a comprehensive article called "Thyroid Disorders and Diabetes" from Diabetes Self-Management, May/June 2001.