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Objective Thyroid hormones affect airway contraction, but the specific effects of thyroid hormones on airways are controversial. Methods We divided 78 advanced-age men with asthma into 3 groups: type I respiratory failure (RF1), type II respiratory failure (RF2), and no respiratory failure (NRF). Pulmonary function was measured after asthma stabilization. Results The free triiodothyronine (FT3) level was significantly higher in the RF1 than RF2 group, but the free thyroxine (FT4), total thyroxine (TT4), and thyroid-stimulating hormone (TSH) levels were not significantly different. In the RF1, RF2, and NRF groups, the correlation coefficients between FT3 and the forced expiratory volume in1 s (FEV 1 ) were 0.317, 0.627, and 0.213; those between FT3 and the FEV 1 /forced vital capacity (FVC) ratio were 0.287, 0.412, and 0.156; those between FT4 and FEV 1 were 0.214, 0.231, and 0.167; and those between FT4 and the FEV 1 /FVC ratio were 0.211, 0.215, and 0.218, respectively. In the RF1 and RF2 groups, the correlation coefficients between the sum activity of peripheral deiodinases (SPINA-GD) and the FEV 1 /FVC ratio were 0.164 and 0.421, and those between SPINA-GD and FEV 1 were 0.284 and 0.491, respectively. Conclusion The correlation between FT3 and pulmonary function is likely caused by changes in thyroid enzymology.