Background: Type 1 diabetes mellitus is an autoimmune disease. Several studies have documented great variations in the prevalence of thyroid dysfunction and autoimmune thyroid disease (AITD) in type 1 diabetic patients. Undiagnosed thyroid dysfunction has negative impact on the metabolic control and will aggravate the cardiovascular disorders. Objectives: We aimed to investigate the presence of thyroid dysfunction and the associated morphological abnormalities in type 1 diabetes mellitus. Methods: 80 type 1 diabetic patients without overt thyroid disease attending the outpatient clinic of diabetes at Kasr Al Aini hospital, faculty of medicine, Cairo University were enrolled in the study. Thyroid functions (TSH, FT4, FT3), anti thyroid peroxidase (anti-TPO) and anti thyroglobulin (anti-TG) antibodies were measured in all patients. Thyroid ultrasound was performed in all patients and in 50 healthy control subjects. The data was analyzed and expressed in terms of mean ± SD. Pearson correlation was performed to establish the relationship between different variables. Results: 52 of 80 patients (65%) showed high TSH levels with mean (12.37±3.9 mIU/ml) and 25 patients (31.3%) showed positive anti-TG anti-TPO levels with mean (906 ± 184.3, 628 ±137.5 IU/ml) respectively. The high TSH levels were statistically significantly associated with high anti-TG levels and anti-TPO levels with (mean 570.23± 372.41, 366.52±281.34 IU/ml) respectively with P-value < 0.001. There was significant increase in the gland volume in diabetic patients with mean (3.4±1.5 ml) versus (2.9±0.9 ml) in the control group, P-value <0.046. Also 25% of patients showed heterogenous hypoechogenic gland texture versus 6 % in the control group which was statistically significantly different, P-value = 0.008 and 50% of the patients showed increase in gland vascularity versus 12% in the control group which was statistically significantly different with P-value <0.001. These morphological abnormalities were associated with high (TSH, anti-TPO and anti-TG) levels but weren't significant. High TSH levels were strongly positively correlated with anti-TPO and anti-TG, r = (0.84, 0.83) respectively, P-value <0.001. Conclusions: Type 1 diabetic patients had high incidence of thyroid dysfunction and AITD associated with morphological abnormalities of the thyroid gland. So we recommend screening for thyroid dysfunction in all patients with type 1 DM to avoid additional cardiovascular risk factors.