CT scans of the thyroid were performed with a Hitachi CT-W2 device. The CT value of the region of interest (ROI) was obtained from CT images of the thyroid. Mean and standard deviation was 112 +/- 10 in 44 normals. Out of 27 cases of hyperthyroidism, the CT value of 13 untreated cases of 58 +/- 4, and that of 13 cases under treatment was 73 +/- 7 in average. The CT value of the single cured case was 100. Twelve cases of chronic thyroiditis and 5 cases of hypothyroidism showed averages of 57 +/- 11 and 36 +/- 13, respectively. The results of the above-mentioned clinical study indicate the estimation of the CT value of the thyroid is useful for diagnosis of various thyroid conditions.