Normal TSH Levels Linked to Increased Risk of Fatal Heart Disease

Research has shown that overt hypothyroidism and hyperthyroidism are linked to an increased risk of coronary heart disease, but a 2008 study out of Norway looked at the impact of thyroid levels considered well within the normal reference range on the risk of fatal heart disease.

More than 25,000 people were included in the study. The study participants were grouped into five different categories according to their Thyroid Stimulating Hormone (TSH) level:

• TSH: less than 0.50
• TSH: 0.50-1.4
• TSH: 1.5-2.4
• TSH 2.5-3.5
• TSH: 3.6 and higher

For the purposes of the study, the TSH reference range was defined as 0.50 to 3.5 mIU/L. (Note: The reference range for laboratories in the United States is typically much wider and runs from approximately 0.50 to 5.5mIU/L. Since 2002, however, some endocrinologists have recommended that the level be narrowed to a range of 0.30 to 3.0.)

During the more than eight-year follow-up of the study, 228 women (1.3%) and 182 men (2.3%) died of coronary heart disease. Of these, 192 women and 164 men had TSH levels within the reference range the researchers used for this study.

The 0.50 to 1.4 TSH range was used as the baseline control group, and as TSH levels rose into the ranges higher than 1.4, the "hazard ratio," which in this case estimated the relative risk of fatal coronary heart disease, also rose in women. (While there was some rise for men, it was not enough to be considered statistically significant.)

The analysis was repeated in another follow-up two years later, and the findings were again consistent with the initial research.

Ultimately, the researchers concluded that there is a clear connection between even low -- but clinically normal -- thyroid function in women and fatal coronary heart disease.

The researchers reported that, to their knowledge, there have not been any clinical trials to determine if thyroid hormone replacement might help protect against coronary heart disease. They did note, however, that other research has shown that thyroid hormone replacement treatment may improve cholesterol levels, improve endothelial function (a measure of vascular disease) and reduce other signs of atherosclerosis.

What This Means for the Public?

Certainly, these findings make it especially important that further gender-specific studies be conducted, with a goal of determining if thyroid hormone replacement for people with TSH levels above 1.4 will reduce the risk of fatal coronary heart disease.

And given that we already know that thyroid hormone replacement can improve various markers for heart disease, there is increasing evidence that the endocrinology and laboratory community need to finally come to some agreement over adopting the recommended narrower reference range.

_Source:

Åsvold, Bjørn MD; et. al. "Thyrotropin Levels and Risk of Fatal Coronary Heart Disease: The HUNT Study," Arch Intern Med. 2008;168(8):855-860. Online