The present study aimed to investigate the occurrence of autoimmune thyroid disease (AITD) in patients with type 1 diabetes mellitus (T1DM) by the presence of antithyroid peroxidase (anti-TPO Ab). Furthermore, we studied the relationship of clinical and thyroid ultrasound (US) characteristics with anti-TPO Ab. This cross-sectional study was performed in Al- Mowasat and Al-Assad University Hospitals from 2021 to 2023. Clinical, laboratories, and US features were collected and analyzed between positive compared to negative anti-TPO Ab. Of 76 patients, anti-TPO Ab was positive in 12 patients (15.7%) with female predominance [n = 10 (83.3%)]. Gender showed a difference between anti-TPO Ab groups (P = .026). DM duration showed a difference (P = .034), which was dominant for a group of positive anti-TPO Ab (median = 9). The age at DM diagnosis also showed a difference (P = .048), where most patients were under 10- years old at diagnosis [n = 39 (51.3%)] and the highest number of anti-TPO Ab positive patients [n = 7 (58.3%)] were in this age category. US findings showed a significant difference (P = .001). Regarding positive anti-TPO Ab patients, the most frequent US finding was immune pattern [7 (58.3%)], which was more common than in the negative group (12.5%). Age, hemoglobin A1c (HBA1c), and body mass index (BMI) did not present differences (P = .391, 0.692, and 0.453, respectively), however, all anti-TPO Ab positive patients were older than ten years and had HBA1c more than 8. Thyroid-stimulating hormone (TSH) was abnormal in 2 patients (16.6%) and both in anti-TPO Ab positive group. This study suggests that anti-TPO Ab appears in older patients and with longer MD duration. Also, data support using US and anti-TPO Ab as earlier markers for AITDs, and further recommending regular annual monitoring by US and anti-TPO Ab in all patients with T1DM for AITDs diagnosis, especially in females.