Busselton study shows subclinical hypothyroidism increases cardiovascular events

Using 2,108 archived serum samples collected from a representative sample of the population in 1981, a cross-sectional analysis identified 119 participants with subclinical hypothyroidism, defined by a TSH level higher than 4.0 mIU/l with normal free thyroxine levels. The age- and sex-adjusted prevalence of coronary heart disease was 80% higher than in euthyroid participants.In a longitudinal study, they examined cardiovascular outcomes until the end of 2001, excluding participants who had CHD at baseline. In people with subclinical hypothyroidism in 1981, there were 21 rather than an expected 9.5 cardiovascular deaths (age- and sex-adjusted hazard ratio 1.5), and 33 compared to 14.7 CHD events (hazard ratio 1.7). The increased risk of CHD events, but not cardiovascular deaths, remained significant after further adjustment for standard cardiovascular risk factors.The increases in risk were very largely confined to participants with a TSH level exceeding 10 mIU/l at baseline. There were no links between subclinical hyperthyroidism and cardiovascular disease. A separate study in two United States cities investigated about 2,700 men and women aged between 70 and 79 years at baseline. During the following four years, people with subclinical hypothyroidism had a higher risk of incident or recurrent congestive heart failure, defined by initiation of treatment, hospitalisation or heart failure-related death. However, there was no link between the thyroid condition and coronary heart disease, stroke, peripheral arterial disease, cardiovascular death or total death.Overt hypothyroidism was known to have marked adverse effects on the cardiovascular system, but it was controversial whether there was a continuous spectrum of effects that extended to subclinical states. Reports of both the studies stressed their findings were observational and did not show whether thyroxine treatment would be beneficial. An editorial noted that moderate elevations of TSH in the presence of normal thyroxine were associated with minimal risk, but trials of thyroxine treatment for severe TSH elevations would be worthwhile.References...