Two studies appearing in the November 28, 2005 issue of the Annals of Internal Medicine suggest that individuals with subclinical hypothyroidism have an increased risk of developing heart failure, and hospitalization or death from coronary artery disease (CAD).
Subclinical hypothyroidism is a condition where the thyroid gland is having difficulty producing enough thyroid hormone. Thyroid hormone helps to regulate the body's metabolism, as well as digestive function, muscle function, and the normal integrity of the skin. Because the thyroid gland is borderline insufficient in subclinical hypothyroidism, the patient's system has to "whip it" to produce enough thyroid hormone. Thus, blood levels of the hormone called thyroid stimulating hormone (TSH,) which stimulates the thyroid gland, are elevated in subclinical hypothyroidism. Since the thyroid gland is indeed making enough thyroid hormone with the extra stimulation provided by high levels of TSH, the patient has no symptoms or signs of hypothyroidism. In fact, the only abnormality seen in this condition is an elevated TSH level. (Given enough time, in most of these individuals the thyroid gland would continue to weaken, and overt hypothyroidism would eventually develop.)
In the two new studies, thousands of patients in the U.S. and Australia agreed to be followed for a period of years, in an effort to evaluate the ability of various risk factors to predict subsequent disease. One of the lab tests measured at the beginning of these studies was the TSH level. Investigators in the U.S. study, the Health, Aging and Body Composition Study, found that patients with elevated TSH levels (at least 4.5 mIU/L) had a significantly higher risk of developing heart failure over the next 4 yeats than patients with normal TSH levels. In the Australian study, the Brusselton Health Study, patients with elevated TSH levels had an increased risk of hospitalization or death from coronary artery disease. In both studies, the higher the TSH level, the higher the risk. The highest risk was seen in patients with TSH levels of 10.0 mIU/L or greater.
DrRich Comments:
Since these are observational studies and not randomized, prospective, controlled trials, they don't actually prove that subclinical hypothyroidism produces heart disease. They just help to establish an association.
Whether subclinical hypothyroidism ought to be treated at all has been controversial, since patients with this condition, in fact, have "normal" levels of thyroid hormone. Still, for patients who have this condition these studies do identify a potentially increased risk for subsequent heart disease, and at the very least ought to stimulate both the doctor and the patient to take special pains to reduce all controllable risk factors for cardiac disease.
Addendum: Thyroid Guide Mary Shomon points out that subclinical hypothyroidism has now been associated with metabolic syndrome, and that new evidence suggests that treating subclinical hypothyroidism may reduce cardiac risk.
