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Life Expectancy Shortened By Subclinical Hyperthyroidism

© Keri

Researchers in Birmingham, UK have determined that a one-time measurement of low serum thyrotropin (TSH) in persons 60 years of age or older is associated with increased mortality from all causes and, particularly, due to cardiovascular disease.

Subclinical hyperthyroidism is a condition without obvious outward effects of hyperthyroidism (too much thyroid hormone production). Individuals with this condition have normal levels of circulating thyroid hormones, T3 and T4, with a low serum thyrotropin (TSH). According to researchers, this condition is common in the elderly. It is also common in those taking thyroxine replacement therapy and those who have been treated for hyperthyroidism.

In an attempt to study the long-term effects of subclinical hyperthyroidism, researchers in Birmingham, UK enrolled a group of 1191 individuals sixty years of age or older. The criteria included only those who did not have known thyroid disease and were not taking any thyroxine replacement. The thyrotropin (TSH) level was measured once at the beginning of the study in 1988-89. The normal range was set at 0.5 - 5.0 mU/L, based on the findings from a control group of 150.

Ten years later, in 1999, a census was taken to determine the status of the participants. 509 of 1191 were deceased, the number only slightly higher than the expected 496 for that population. Over a ten year period the numbers were nearly consistent with expectations for age, sex, and year for England and Wales.

On closer analysis, it was found that there was a dramatic difference in the first five years following the initial measurement. During this time, overall deaths were significantly higher in the group with subclinical hyperthyroidism, particularly in the area of cardiovascular disease. These findings were similar for men and women. Interestingly, those with elevated or normal TSH showed no significant difference from the expected death rate and cause of death.

What causes this difference? It is known that overt hyperthyroidism has a profound effect on the cardiovascular system, including rapid heart rate from atrial fibrillation. Atrial fibrillation increases the risk of clot formation that can lead to stroke or heart attack. In itself, atrial fibrillation is considered a risk factor for heart disease. Hyperthyroidism also increases cholesterol levels thus increasing the risk of cardiovascular disease (hardening of the arteries). The older one is the more profound these effects may be.

Based on these findings, researchers have suggested that those with subclinical hyperthyroidism should be considered for treatment to reduce the risk of heart disease and early mortality.

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