Effects of severity of chronic obstructive pulmonary disease on thyroid function

To investigate thyroid function in chronic obstructive pulmonary disease (COPD), 46 consecutive patients (35 men) with stable, mild-to-severe disease, having a mean (SD) age of 67 +/- 7 years were studied. All subjects underwent pulmonary function tests (PFTs), arterial blood gas determination, and measurement of serum total thyroxine (TT4), total triiodothyronine (TT3), resin T3 uptake (RT3U), reverse triiodothyronine (rT3), and thyroid-stimulating hormone (TSH) levels. The free thyroxine and free triiodothyronine indexes (FT4I = RT3U/30TT4 and FT3I = RT3U/30TT3, respectively) along with the TT3/TT4 ratio were calculated; the latter was used as a marker of peripheral conversion of thyroxine into triiodothyronine. Interleukin (IL)-6 was also measured to evaluate its potential associations with thyroidal hormone levels. On the basis of forced expiratory volume in 1 second (FEV1), patients were divided in 2 groups: group 1, (FEV1 > or = 50% of predicted, n = 26), with mild-to-moderate COPD and group 2 (FEV1 < 50% of predicted, n = 20) having severe disease. All subjects had normal serum thyroid hormone levels; for the entire COPD population, mean values were 7.80 +/- 1.60 microg/dL for TT4, 1.12 +/- 0.20 ng/mL for TT3, 29.0 +/- 1.88 for RT3U, 7.54 +/- 1.34 for FT4I, 1.07 +/- 0.16 for FT3I, 18.71 +/- 5.89 ng/dL for rT3, and 1.15 +/- 0.6 microU/mL for TSH. Mean TT3/TT4 ratio was 0.14 +/- 0.03. In group 1, TT3, TT4, and TT3/TT4 ratio did not correlate with age, FEV1, PaO2, or inhaled corticosteroids. Similarly, in group 2, TT3 and TT4 were unrelated to the above-mentioned variables; however, there was a strong positive correlation between TT3/TT4 ratio and PaO2 (r =.61, P =.004). IL-6 was within normal limits in all subjects, and it did not correlate with any thyroid hormone either in group 1 or in group 2. It is concluded that in stable COPD, severity of disease through hypoxemia is important in determining the peripheral metabolism of thyroid hormones. Whether this constitutes an adaptation is not known.