Objective: To explored the thyroid function and prevalence of thyroid diseases in primary aldosteronism (PA) and PA subtypes. Design and method: Patients with PA and Essential hypertension (EH) in our hypertension center between August 2011 to June 2022 were enrolled in this observational study. All PA patients underwent adrenal venous sampling (AVS), thus receiving the most accurate subtype diagnosis available. The differences of thyroid function and the prevalence of thyroid diseases between PA and EH patients were analyzed. Results: 7161 participants were included in the study with a mean age of 48.20±8.83 years and a mean BMI of 26.52±3.45 kg/m2. Two groups: PA patients showed lower FT4, FT3, TT4, TT3, positive TPOAb and higher prevalence of thyroid nodules (22.5%) than EH patients. PA was significantly associated with thyroid nodules (OR: 1.245, 95%CI: 1.052-1.473, P= 0.011). Three groups: statistical differences in FT4, FT3, TT4, TT3, and positive TPOAb levels were found in APA, IHA and EH. Thyroid nodules are most prevalent in APA (26.00%), followed by IHA (20.80%) and EH (17.80%). APA showed a significant association with thyroid nodules (OR: 1.345, 95%CI: 1.015-1.781, P=0.039). Additionally, it was found that FT4, FT3, TT4, and TT3 were negatively correlated with the PAC levels in patients. The stratification analysis showed that the prevalence of thyroid nodules was higher in subjects with PAC leve greater than or equal to 12ng/dL. Conclusions: Compared to EH groups, PA patients had lower thyroid function and higher prevalence of thyroid nodules.