The sleep apnea syndrome (SAS) and hypothyroidism have several common signs. This similarity creates a significant risk of undiagnosed hypothyroidism during the SAS. We studied the interest of thyroid function testing at patients suspected of having SAS. 201 consecutive patients, having a suspicion of SAS without history of hypothyroidism, underwent polysomnography (PSG) and biochemical thyroid testing. 91 patients (47%) had a SAS. Three patients among them (1.5% of patients undergoing PSG or 3.3% of those having a SAS) had previously undiagnosed hypothyroidism. These three patients have been treated by thyroxine. After euthyroidism was obtained by the treatment, SAS disappeared in 2 patients. The prevalence of the hypothyroidism in patients suspected of having SAS is not different from the one in the general population. However it is greater in patients with SAS. So we can conclude that biochemical screening for hypothyroidism is indicated only in case of proven SAS, in order to distinguish between a primary and a secondary origin.