Background: Subclinical hypothyroidism (SCH) is defined biochemically as a high serum thyroid-stimulating hormone (TSH) and normal serum free thyroxine (T4) and triiodothyronine (T3) concentrations. SCH influences cardiovascular system and metabolic parameters, and the effect on the respiratory system is controversial. The aim of this prospective observational case-control study was to evaluate the pulmonary function of patients diagnosed with SCH to find out lung function impairment if any. Methods: The study comprised of 145 participants (70 patients with SCH and 75 healthy controls) in the age-group of 18-55 years. Pulmonary function tests were performed by using the computerized spirometer. The parameters of the two groups were compared by student’s t-test and P <0.05 was considered statistically significant. Results: The patients of SCH showed a highly significant reduction in Forced Vital Capacity (FVC), Forced Expiratory Volume in one second (FEV1) and their predicted percentages as compared to healthy controls (P<0.001). Forced Expiratory Flow 25-75 (FEF25-75) and its predicted value was also significantly lower in patients than in controls (P<0.05), but the Peak Expiratory Flow Rate (PEFR) and the difference in values of FEV1/FVC% values between two groups was not statistically significant. Conclusion: Pulmonary functions were affected in the patients with SCH. A mixed pattern of respiratory dysfunction was seen in patients with SCH. Therefore, pulmonary functions should be evaluated in SCH patients to detect early respiratory dysfunction.
