Aim: Subclinical hypothyroidism reflects the earliest stage of thyroid dysfunction with subjects having normal or decreased fT4, normal fT3 and decreased thyroid stimulating hormone levels. Thyroid hormones have effects on the respiratory system functions. It is well known that thyroid hormones have effects on the respiratory system functions however there are few studies concerning the effect of subclinical hypothyroidism on the respiratory system. We aimed to investigate the respiratory functions in patients diagnosed with subclinical and clinical hypothyroidism. Material and Methods: A total of 61 patients with clinical (n=31) or subclinical (n=30) hypothyroidism and 27 healthy subjects as a control group were included in the present study. Respiratory functions were evaluated by simple spirometry. All respiratory parameters including FVC, FVC%, FEV1, FEV1%, FEV1/FVC, FEF25-75, FEF25-75%, PEF, PEF% were assessed. Additionally, free triiodothyronine (FT3), free thyroxin (FT4) and thyroid stimulating hormone (TSH) levels were measured in all subjects. Results: Patients in the clinical hypothyrodism, subclinical hypothyroidism and control groups were homogenous in terms of age and gender distribution. Body mass index (kg/m2) was determined to be significantly higher in patients with clinical hypothyroidism when compared to control subjects (p<0.05). Spirometric evaluation of the respiratory function revealed significantly lower values for FVC (L; p<0.05), FVC (%; p<0.05), FEV1 (L; p<0.05) and FEV1 (%; p<0.05) in all hypothyroid patients regardless of the clinical type when compared to control subjects. Conclusions: Respiratory function tests seem to be beneficial in the evaluation of respiratory functional state in hypothyroidism. We think that community screening with simple spirometry may be helpful for patients with risk of subclinical hypothyroidism.