TNF and chronic obstructive pulmonary disease: In the present study, we found that the serum levels of IL-6, TNF-α, RANKL and the ratio of RANKL/OPG were significantly higher in the COPD with low BMD group than in the other two groups (controls and normal BMD); the level of IL-1β was also higher in the low BMD group than in the normal BMD group, but did not reach statistical significance.