The association of overt hypothyroidism with cardiovascular disease (CVD) is widely accepted with the implicit assumption that the risk of CVD rises as the severity of hypothyroidism increases. There are several possible mechanisms by which hypothyroidism could cause CVD, including increases in the probability of atherosclerosis by an adverse lipid or homocysteine profile, as well as direct adverse effects on myocardial or cardiovascular function.
In contrast, the association of subclinical hypothyroidism (defined as a serum-free thyroxine level within the reference range and an elevated serum thyroid-stimulating hormone [TSH] level) with CVD in studies to date has been controversial. Community-based, cross-sectional studies of the relationship between subclinical hypothyroidism (SH) and the prevalence of CVD at study entry have produced mixed results, with some showing a strong association,1 some a weak association,2,3 and others no association.4,5 Longitudinal studies1,3,6- 8 of the relationship between SH at study entry and the incidence of new CVD developing during subsequent follow-up have shown a minimal association. Such conflicts may arise from differences in the patient populations studied with variations in selection criteria such as age, sex, and race distribution as well as duration of follow-up.
Crapo LM. Subclinical Hypothyroidism and Cardiovascular Disease. Arch Intern Med. 2005;165(21):2451-2452. doi:10.1001/archinte.165.21.2451