Hypothyroidism is associated with depression of hypoxic and hypercapnic ventilatory drives, resulting in alveolar hypoventilation (pCO2>45 torr). There is dearth of studies describing prevalence of hypoventilation in obese hypothyroid patients undergoing nocturnal polysomnography (NPSG). This is a retrospective study of consecutive obese (BMI ≥ 30 kg/m2) adult hypothyroid subjects who underwent NPSG between November 2015 and October 2016. Data collection included demographics, NPSG variables, end-tidal pCO2 (PetCO2), peak CO2, % of total sleep time with PetCO2 >50 torr (pCO2>50), oxygen saturation (SpO2) nadir, average sleep SpO2, comorbidities, and thyroid stimulating hormone (TSH). We used Chi-square test and Wilcoxon’s rank order test for statistical analysis. We identified 53 subjects: 18 with and 35 without alveolar hypoventilation. The demographic variables reported as median (quartile1, quartile3) were: age 58 (47, 68) years, BMI 37.8 (34.1, 44.3), female 66%. The variables which were significantly associated with hypothyroid subjects with alveolar hypoventilation compared to hypothyroid subjects without alveolar hypoventilation were: AHI 40 (19.5,69.7) vs 18.6 (7.7,30) [p=0.038], SpO2 nadir: 79% (73%,83%) vs 86% (80.5%,88.5%) [p=0.018], average sleep SpO2: 92% (90%,94%) vs 95% (94%,96%) [p=0.0076], TSH: 2.7 (1.5,7.9) vs 1.28 (0.04,2.36) [p=0.03], peak CO2: 59.5 (57,62) vs 50 (45,52.5) [p<0.0001] and pCO2>50: 57% (24%,84%) vs 0% (0%,1%) [p<0.0001]. Uncontrolled hypothyroidism was present in 36.4% of the subjects with hypoventilation as compared to 9% of those without hypoventilation, along with a higher percentage of the following comorbidities: hypertension (78% vs 71%), heart failure (39% vs 11%), depression (27% vs 22%) and diabetes (72% vs 31%), with only diabetes being statistically significant [p=0.004]. Adult obese hypothyroid subjects with alveolar hypoventilation have lower average sleep oxygen saturation, lower oxygen saturation nadir, and higher AHI compared to those without hypoventilation. None.