Background; Type 1 diabetes (T1DM) is an autoimmune disease often associated with thyroid abnormalities. This study was aimed to assess the thyroid function in pediatric patients with T1DM and it's relation to diabetes severity. Subjects and methods: This is a cross section study was performed at Zagazig Univeristy hospital in diabetes clinic and Pediatric Diabetes Insurance Clinic in Sharkia Governorate,on two 212 children with documented T1DM, during a period from November 2019 to April 2020. Patients were divided according to thyroid function tests; patients with T1DM and hypothyroidism and patients with T1DM and normal thyroid function. All patients have been subjected to full history taking, Family history of Diabetes Mellitus, Thyroid disease and consanguinity, Medications including required insulin dose, Occurrence of Diabetic Ketoacidosis (DKA) at first presentation and complications). Complete clinical examination and Investigations of the Hemoglobin A1C (HbA1C), Thyroid function tests (TSH, Ft4) Antithyroid antibodies (Tg Ab). Results: Children who had both T1DM and hypothyroidism most of them experienced DKA as the first presentation of their disease (68%) in comparison to (36.3%) in diabetic patients with normal thyroid function. Additionally children with T1DM and hypothyroidism was experienced a more aggressive disease; in comparison to children with T1DM who had normal thyroid function, children who had both T1DM and hypothyroidism had significantly higher HbA1C levels at enrolment (11.37 ±2.10). Conclusion: Hypothyroidism is prevalent among pediatric patients with T1DM and is associated with higher rates of DKA and the disease severity.