Can "Low-Normal" Thyroid Function Lead to Heart Disease?

A study published this week in the Archives of Internal Medicine suggests that women who have very mild hypothyroidism (i.e., an underperforming thyroid gland) may be at increased risk for heart disease.

The study looked at levels of Thyroid Stimulating Hormone (TSH) in over 17,000 women from Norway, and found that over an 8-year period, women whose TSH levels were in the "high normal" range had significantly higher cardiovascular risk than women whose TSH levels were lower.

TSH is a hormone that sets the throttle for the thyroid gland; that is, if the thyroid gland is not producing enough thyroid hormone, TSH levels increase to make the thyroid gland work harder. So, elevated TSH levels indicate that the thyroid gland is underperforming (so it needs to be "whipped"). In fact, elevated levels of TSH is the best way to diagnose hypothyroidism.

The so-called "normal range" for TSH levels is already controversial. Many have pointed to evidence that what most now consider to be the normal range for TSH is actually too wide, and that people with TSH levels in the "high-normal" range (which generally indicates "low-normal" thyroid gland function) should actually be considered as frankly hypothyroid. This new study certainly adds fuel to that argument.

This study does not prove that "high-normal" TSH levels actually cause an increase in cardiac risk, but it does suggest a correlation. A clinical trial will be needed to test the proposition that treating "high-normal" TSH levels in women (by giving thyroid hormone) might reduce cardiac risk.

For an in-depth discussion of this issue, read this article by Mary Shomon, the About.com Guide to Thyroid Disease.