The purpose of the present study is to investigate by a computed tomography (CT) thyroid CT number (Hounsfield units; H.U.) in chronic thyroiditis (CH). The correlation of the CT value with the concentration of KI solution in the test tube of thyroid phantom for thyroid scintigram was linear. No significant correlation between thyroid CT number and the urinary iodine concentration was also observed in 36 patients with CH. A mean thyroid CT number (+/- SD) of 81 +/- 24 H.U. in 155 patients with CH was significantly (P < 0.001) lower than 125 +/- 18 H.U. in 95 normal subjects. The patients with CH could be arbitrarily divided into 3 groups according to the decrease in their thyroid CT number: normal (N) group (> or = 88 H.U.), moderately decreased (MD) group (70-87 H.U.) and the greately decreased (GD) group (< 70 H.U.). There was a significant difference in the thyroid volume among 21.2 +/- 9.1 cm3 in the N group, 32.2 +/- 18.1 in the MD group (P < 0.05 vs. N group) and 43.6 +/- 23.3 in the GD group (P < 0.01 vs. N and MD group). The mean serum TSH concentration of 38.95 +/- 54.61 microU/ml in the SD group was significantly (P < 0.1) higher than either 4.11 +/- 3.64 in the MD group or 2.00 +/- 1.43 in the N group. The serum TSH level in the MD group differed significantly (P < 0.05) from that of the N group.(ABSTRACT TRUNCATED AT 250 WORDS)