Association of free thyroxine with obstructive lung pattern in euthyroid middle-aged subjects: A population-based study

BACKGROUND: The association between thyroid hormone levels and pulmonary function in euthyroid population is still unclear. We aimed to examine the relationship between thyroid function and lung function in a large cohort study of euthyroid subjects. METHODS: We analyzed biochemical and spirometry data from a nationwide, population-based, cross-sectional survey (KNHANES VI). A total of 1,261 middle-aged participants aged 45–65 years with spirometry tests and normal thyroid function were included in this study. The subjects were grouped according to free thyroxine (fT4) (ng/dL) quartiles (Q1, 0.89–1.09; Q2, 1.10–1.19; Q3, 1.20–1.30; Q4, 1.31–1.76). Obstructive lung pattern was defined as forced expiratory volume in one second (FEV(1))/forced vital capacity (FVC) <0.7. The probability of obstructive lung patterns according to fT4 quartiles was assessed using logistic regression models, adjusted for potential confounders. RESULTS: Overall, 10.9% of the subjects had an obstructive lung pattern. The mean fT4 levels were significantly higher in those with obstructive lung pattern than in those with normal lung function (1.26 vs. 1.20 ng/dL, p<0.001). The proportion of participants with obstructive lung pattern increased across the fT4 quartile categories (p<0.001). With the Q1 group as reference, the multivariate-adjusted odds ratios (95% confidence intervals) for obstructive lung pattern in the Q3 and Q4 groups were 2.875 (1.265–6.535) and 2.970 (1.287–6.854), respectively, even after adjusting for confounding variables. CONCLUSION: High fT4 levels are an independent predictor of obstructive lung pattern in euthyroid middle-aged subjects. Further prospective studies are needed to confirm these findings.