Objective: Autoimmune thyroiditis (AIT) is the most common autoimmune thyroid disease. Longitudinal relaxation time mapping (T1-mapping) measured by MRI is a new technique for assessing interstitial fibrosis of some organs, such as heart and liver. This study aimed to evaluate the relationship between T1-mapping value and thyroid function and determine the usefulness of T1-mapping in identifying thyroid destruction in AIT patients. Methods: This case-control study recruited 57 drug-naive AIT patients and 17 healthy controls. All participants were given thyroid MRI, and T1-mapping values were measured using a modified look-locker inversion-recovery sequence. Results: AIT patients had significantly higher thyroid T1-mapping values than the healthy controls (1.077 +/- 177 vs 778 +/- 82.9ms; P< 0.01). A significant increase in thyroid T1-mapping values was presented along with the increased severity of thyroid dysfunction (P< 0.01). Correlation analyses showed that increased thyroid T1-mapping values were associated with higher TSH and lower FT3 and FT4 levels (TSH: r= 0.75; FT3: r= -0.47; FT4: r= -0.72; all P<0.01). Receiver-operating characteristic curve analysis revealed a high diagnostic value of T1-mapping values for the degree of thyroid destruction (area under the curve was 0.95, 95% Cl: 0.90-0.99, P<0.01). Conclusions: AIT patients have higher thyroid T1-mapping values than the healthy controls, and the T1-mapping values increased with the progression of thyroid dysfunction. Thyroid T1-mapping value might be a new index to quantitatively evaluate the degree of thyroid destruction in AIT patients.