Thyroid function test (TFT) impairments can be detected in extrathyroidal dysfunction, primarily in chronic obstructive pulmonary disease (COPD) with acute respiratory failure (RF). The aims of this study were to: (i) evaluate TFT impairments in patients with RF, (ii) compare TFT results to a control group without RF and (iii) assess the effects of thyroid dysfunction on clinical outcome and prognosis of RF. The TFT parameters were assessed in 65 patients (65.0 +/- 10.0 years, 49 males) with RF and compared to 18 patients (64.4 +/- 9.8 years, 13 males) with lung disease and no RF (p> 0.05). Arterial blood gas analysis, free T3 (FT3), free T4 (FT4) and TSH levels were all measured. The impairments of TFT were demonstrated in 34 (52.3%) patients with RF and 8 (44.4%) patients without RF (p> 0.05). The most common finding was a decrease in at least one of the TFT parameters in both groups (43.1% vs. 44.4%, respectively). In RF group, there was no significant association between TFT results and gender, age, diagnosis and co-morbid disease. However, need for invasive mechanical ventilation was higher both in patients with low FT3 and low FT4 when compared to those with normal TFT results (p= 0.001 and p= 0.003, respectively). In-hospital mortality rate was also higher both in the patients with low FT3 and low FT4 than the others (p= 0.006 and p= 0.01, respectively). We conclude that TFT impairments are not observed more frequently in patients with RF when compared to the patients without RF. However, low FT3 and FT4 levels increase the rates of invasive mechanical ventilation and mortality.