Objective: Obstructive sleep apnea-hypopnea syndrome (OSAHS) is closely associated with hypothyroidism. This study aimed to evaluate the prevalence and correlation of hypothyroidism in patients with OSAHS. Methods: This investigation included a cohort of 1693 patients newly diagnosed with OSAHS, all of whom underwent polysomnography and thyroid function assessments. Results: The study cohort comprised 1693 OSAHS patients (60.4% male). No significant differences in thyroid-stimulating hormone (TSH) or free thyroxine (FT4) levels were observed across OSAHS severity groups (p = 0.670 and p = 0.307). Notably, patients with overt hypothyroidism demonstrated significantly lower lowest oxygen saturation (LSpO2) levels than their euthyroid counterparts (p = 0.035). The analysis did not reveal any significant correlations between TSH/FT4 levels and the apnea-hypopnea index, mean oxygen saturation (MSpO2), or LSpO2. Nevertheless, TSH levels were positively correlated with female sex and age (p = 0.001), while no correlation was found with body mass index. After controlling for sex and age, TSH was positively correlated with apnea-hypopnea index (p = 0.002) and negatively correlated with both MSpO2 and LSpO2 (p = 0.001). In contrast, FT4 levels exhibited a negative correlation with apnea-hypopnea index (p = 0.016) and positive correlations with MSpO2 and LSpO2 (p = 0.030 and p = 0.018). The overall prevalence rates of hypothyroidism and overt hypothyroidism within the study population were determined to be 13.7% and 3.8%, respectively, with no significant differences observed across the severity categories of OSAHS (p = 0.166 and p = 0.193). Subclinical hypothyroidism was identified in 9.9% of the patients, with a notably lower prevalence in those with severe OSAHS compared to those with milder forms of the condition (p = 0.004). Conclusion: In patients with OSAHS, 13.7% had hypothyroidism, with 3.8% being overt hypothyroidism and 9.9% being subclinical hypothyroidism. Thyroid function parameters are associated with the severity of OSAHS and are influenced by sex and age.