Howard LeWine, M.D., is chief editor of Internet Publishing at Harvard Health Publications. He is recognized as an outstanding clinician and teacher and is a recipient of the Internal Medicine Teacher of the Year award at Brigham and Women's Hospital. Dr. LeWine continues to practice Internal Medicine; most recently he became a hospitalist after practicing primary care for over 20 years.

Question:

What role does low thyroid hormone play in high blood sugar and cholesterol?

Answer:

Low thyroid hormone production by the thyroid gland (hypothyroidism) can cause a significant elevation in blood cholesterol levels. The relationship between blood sugar levels and thyroid hormone is not as straightforward.

Untreated hypothyroidism causes a rise in total cholesterol and especially LDL ("bad") cholesterol. Treatment with levo-thyroxine or other thyroid medication will lower cholesterol levels. But even with adequate treatment of the thyroid condition, there may be other reasons for your cholesterol to remain above normal, such as diet and genetic factors.

The relationship between blood sugar and thyroid hormone depends on the underlying conditions you may have. If high blood sugar is related to type 1 diabetes there is a 30% chance you also have an underactive thyroid gland and low thyroid hormone.

Most cases of type 1 diabetes and hypothyroidism have a similar cause — an autoimmune reaction. Your body produces antibodies that attack the pancreas and thyroid gland. The pancreas stops making insulin and blood sugar goes up. The damaged cells in the thyroid gland can't produce thyroid hormone.

A different situation relates to the effects of thyroid hormone on the rate of metabolism. Too much thyroid hormone speeds up metabolism and tends to raise blood sugar. A person might need more insulin or other diabetes medications to keep blood sugars controlled. With a low thyroid level, less insulin might be required.

While your situation of high blood sugar and low thyroid hormone doesn't fit what the textbooks say, this is often how things happen in real life. So your goal should be to work with your doctor to get your thyroid blood level into the normal range and keep it there. Then it should be easier to control your blood sugars with diet, exercise and medications.