Association of low baseline free thyroxin levels with progression of coronary artery calcification over 4 years in euthyroid subjects: The Kangbuk Samsung Health Study

Objective Overt and subclinical hypothyroidism are risk factors for atherosclerosis and cardiovascular diseases. It is unclear whether thyroid hormone levels within the normal range are also associated with atherosclerosis measured by coronary artery calcium (CAC). Context This study aimed to examine the relationship between normal variations in thyroid function and changes in CAC. Measurements We conducted a 4-year retrospective study of 2173 apparently healthy men and women with normal thyroid hormone levels. Their free thyroxin (FT4), free triiodothyronin (FT3) and thyroid-stimulating hormone (TSH) levels were measured by electrochemiluminescent immunoassay. The CAC score (CACS) of each subject was measured by multidetector computed tomography in both 2010 and 2014. Progression of CAC was defined as a CACS change over 4 years > 0. Results The mean CACS changes over 4 years by quartiles of baseline FT4 level (lowest to highest) were 12·9, 8·43, 7·82 and 7·81 (P = 0·028). CAC progression was not significantly associated with either the baseline FT3 or TSH levels. The odds ratios (OR) for CAC progression over 4 years (highest vs lowest quartile for baseline FT4) were 0·647 (95% confidence interval (CI) 0·472-0·886) after adjustment for confounding factor, which were attenuated with further adjustment for lipid profiles, homoeostasis model assessment of insulin resistance, high-sensitivity C-reactive protein and hypertension [0·747 (95% CI 0·537-1·038)]. Quartiles of baseline FT3 or TSH level did not show any increased OR for CAC progression after adjustment for confounding factors. Conclusions In this cohort of euthyroid men and women, a low baseline FT4 level was associated with a high risk of CACS progression over 4 years.