BACKGROUNDThe aetiology of thyroid nodules is diverse. Benign causes include the colloid nodule and the classical multinodular goiter. Occasionally, Hashimoto's thyroiditis and Grave's disease may present with nodularity. Malignant causes include thyroid cancer, lymphoma as well as metastasis to the thyroid glands. The objectives of the study were to compare ADC value of indeterminate thyroid nodules (includes TIRADS 3, 4 & 5) with their histopathology and then evaluate its role in differentiating malignant from benign thyroid nodules. MATERIALS AND METHODSThe prospective study was conducted in Department of Radiology and Imaging Sciences, Malabar Institute of Medical Sciences Ltd, Calicut, Kerala between 1 st October 2015 to 30 th March 2017, for a period of 18 months. The study population for analysis is the patients undergoing ultrasound thyroid, detected to have TIRADS 3, 4 and 5 lesions. All patients presenting with indeterminate thyroid nodule on ultrasonography in our hospital are subjected to MRI DW sequences the ADC was calculated. RESULTSA total of 80 patients with USG diagnosis of indeterminate thyroid nodule (TIRADS 3, 4 & 5) that came to the department before undergoing surgery were included in the study. For all these patients, T2 weighted MRI, diffusion weighted imaging and ADC mapping was done. Histopathology findings of each patient is collected and correlated with MRI findings to finalize the diagnosis. The sensitivity and specificity for various ADC values were calculated from ROC curve and it was noted the best ADC value for differentiating benign from malignant thyroid nodules according to our study is 1.745 with the highest sensitivity and specificity to qualify it as a screening test. The positive predictive value and negative predictive value when taking 1.745 as cut off ADC are 89.5% & and 98.4% respectively. The mean ADC of the malignant thyroid nodules (1.52± 0.23 x 10 -3 mm 2 /s) was significantly lower than that of the mean ADC of the benign thyroid nodules (2.25± 0.41 x 10 -3 mm 2 /s). Range of mean ADC value for benign lesions was 1.56 -3.33 and for malignant lesions was 0.96 -1.87. The proportion of cases with malignancy increases with decreasing ADC value. 17 out of 18 malignant cases were having an ADC value of less than 1.745 (Sensitivity -94.4%). CONCLUSIONThe study showed MRI with diffusion weighted imaging and ADC mapping is a promising tool in the armamentarium for the differentiation of benign and malignant thyroid nodules.