The thyroid is a small gland in the neck that releases hormones that control how fast the cells in the body perform their functions. It has long been known that if this gland is obviously under-active, there is often an accompanying raised blood cholesterol level and high blood pressure, and an increased risk of heart and blood vessel disease. A recent study has shown that, quite often, elderly women have reduced activity of their thyroid that is not clinically obvious - it can only be detected by a blood test. But can this "subclinical" under- activity lead to cardiovascular disease?

The Rotterdam Study is a large study of older persons living in a suburb of the Dutch city. Between 1990 and 1993 all persons 55 or over were invited to take part, and about 8,000 accepted. Over 1,000 women who had undergone menopause were enrolled for this thyroid gland study. Information was collected that included height, weight, smoking status, medication history, blood pressure, an ECG, blood cholesterol, and blood tests of thyroid gland function (thyroid stimulating hormone = TSH, thyroxine, and antibodies to thyroid cell enzyme).

A TSH level above 4.0 mU/L together with a normal thyroxine level indicated "subclinical" under-activity of the gland. (If the THS was above 4.0 mU/L and the thyroxine level was decreased, the gland was quite clearly under-active, and this would be associated with obvious signs such as feeling cold, loss of eyebrows, coarse hair, dry scaling skin, and lethargy - click for more on this).

Radiography of the lower back was done to demonstrate any small deposits of calcium in the aorta (the main arterial trunk), which would indicate atherosclerosis (the narrowing or blockage of arteries that starts with fat deposits in the artery wall). Thorough questioning and an ECG allowed the investigators to determine if the participant had had a previous heart attack (myocardial infarction, MI).

After all these investigations at baseline, the participants were followed carefully up to mid-1996. Information on fatal and nonfatal MIs was gathered from the family physicians, and verified by checking the patients' medical records.

There were over 1,000 women who had normal thyroid activity or "subclinical" thyroid under-activity at baseline. Their average age was 69 years. Almost 12% of them were found to have "subclinical" under-activity of the gland.

The women with "subclinical" under-activity of the gland were 1.7-times more likely to have atherosclerosis of their aorta and 2.3-times more likely to have had an MI at the time of examination at baseline.

During the follow-up period, which averaged 4.5 years, 16 women experienced an MI for the first time. Analysis showed that women with "subclinical" thyroid under-activity were 2.5-times as likely to experience an MI.

The authors did further analyses which showed that "subclinical" thyroid under-activity carried roughly a similar risk of association with MI as smoking, high blood cholesterol, high blood pressure and diabetes.

This study shows that "subclinical" under-activity of the thyroid gland is quite common in elderly women, and is associated with atherosclerosis of the aorta and heart attack. It is clear that this thyroid condition must be added as an equal partner to the better-known risk factors for these cardiovascular diseases - smoking, high cholesterol, diabetes, and hypertension.