Patients with obstructive sleep apnea hypopnea syndrome (OSAHS) and thyroid function abnormalities, such as hypothyroidism and Hashimoto's thyroiditis, usually have closely resembling clinical features. Differentiation between these disorders is made more difficult because hypothyroid patients are also at risk for secondary sleep-disordered breathing. The aim of our study is to evaluate the prevalence of thyroid function abnormalities in children with OSAHS. Forty-four children (15 females: 29 males), 2.5-14.5 (7.43±2.98) years old were studied with overnight polysomnography. Biochemical screening of thyroid gland function was also carried out. Patients were judged to have OSAHS based mainly on the evaluation of Apnea Hypopnea index per hour of sleep (AHI). 15/44 (34.1%) children had mild OSAHS, 17/44 (38.6%) moderate and 12/44 (27.3%) severe OSAHS. Hypothyroidism was recorded only in 5/44 (11.4%) and Hashimoto's thyroiditis in 3/44 (6.8%) of OSAHS patients. Two patients with hypothyroidism showed mild and three severe OSAHS, while from the 3 children with Hashimoto's thyroiditis one presented mild, one moderate and one severe degree of OSAHS. Although the majority of studies in bibliography worldwide do not consider necessary the systemic evaluation of thyroid gland function in patients with breathing disorders during sleep, it seems that in children this type of screening is required for the differential diagnosis between primary sleep apnea and hypothyroid sleep-disordered breathing in order to differentiate these two conditions. Therefore, the laboratory investigation of thyroid gland function could be considered necessary.