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We determined the CT density of the thyroid gland in 60 patients with normal thyroids and 176 patients with various thyroid disorders. The density was 75 +/- 6.2 H.U. in normal thyroids, which was markedly higher than in goiters (66 +/- 6.0 H.U.), while there was a considerable further decrease in patients with immunogenic hyperthyroidism, the density being 48.5 +/- 7.9 H.U. The thyroids of patients with nonimmunogenic hyperthyroidism differed from these by virtue of a significantly greater density, 79.8 +/- 12.5 H.U. The clinical importance of CT investigation of the thyroid is its ability to distinguish rapidly between immunogenic hyperthyroidism without ocular symptoms and the (mostly iodine induced) non-immunogenic form. There is a linear correlation between CT density and iodine concentration in the thyroid tissue; this was determined in surgical specimens from 17 patients. Iodine concentration in the thyroid, as well as the iodine content of the whole gland, can thus be calculated from the measured CT density at any time by estimating the volume of the gland with ultrasound and combining this value with the measured iodine concentration. The results correlate well with those found using the x-ray fluorescence method.