Low thyroid without symptoms increases heart risk

The results of a study of older adults suggest that people who have an under active thyroid, or hypothyroidism, but with no symptoms, have an increased the risk of congestive heart failure, but not other cardiovascular disease or death.

Asymptomatic hypothyroidism is characterized by an increased level of thyroid stimulating hormone (TSH), but a normal level of thyroxine (T4). The prevalence of this condition increases with age, Dr. Nicolas Rodondi from the University of Lausanne, Switzerland and colleagues explain in the November 28th Archives of Internal Medicine.

Asymptomatic hypothyroidism has been associated with higher levels of some cardiac risk factors but data on cardiovascular outcomes and death are limited, they also note.

Rodondi's team therefore examined the risks of heart failure, coronary heart disease, stroke, peripheral artery disease and death compared with TSH levels in a 4-year study of 2,730 men and women between 70 and 79 years old.

Hypothyroidism was detected in 338 (12.4 percent) of study participants.

Compared with adults with normal thyroid function, congestive heart failure was about twice as common in patients with moderate or severe asymptomatic hypothyroidism.

The researchers defined moderate asymptomatic hypothyroidism as a TSH level of 7.0 to 9.9 mIU/L and severe asymptomatic hypothyroidism as a TSH of 10 mIU/L or greater. Compared with study participants with normal thyroid function, older adults with TSH levels of 7.0 mIU/L or higher had a two- to three-fold increased risk of congestive heart failure. The rate of new or recurrent heart failure was also increased in this group.

Heart failure was not increased among those with TSH levels between 4.5 and 6.9 mIU/L, which indicated mild asymptomatic hypothyroidism.

The condition was also not associated with an increased risk of coronary heart disease, stroke, peripheral arterial disease, or cardiovascular and total mortality.

In comments to Reuters Health, Rodondi noted that the results need to be confirmed by other large studies and in younger patient populations. It also remains to be seen if symptomatic hypothyroidism causes or worsens heart failure that is pre-existing.

It's still controversial, Rodondi added, whether screening and treatment of asymptomatic hypothyroidism is advisable because the current evidence is limited by the small number of large studies that have been conducted.

In a related editorial, Dr. Lawrence M. Crapo of Santa Clara Valley Medical Center in San Jose, California, says that this study supports the idea that treating severe cases of asymptomatic hypothyroidism with levothyroxine in patients younger than 80 years may be helpful, but this need to be confirmed in a therapeutic trial.

SOURCE: Archives of Internal Medicine, November 28, 2005.