The thyroid gland comprises numerous follicles, each containing thyroid follicular cells arranged in a circular pattern surrounding a colloid containing thyroid hormones produced from thyroglobulin and iodide. The secretion of the thyroid hormone is regulated by TSH [7]. On CT images, the CT value of the thyroid gland is approximately 100–110 HU, higher than that of soft tissues [1, 2]. This elevation in CT value may be attributed to the presence of several colloids containing iodine in the thyroid gland. Notably, a strong correlation has been reported between the iodine concentration in the thyroid tissue and CT value [1]. In the present study, CT value exhibited a significant positive correlation with EHL, which is the rate of iodine accumulation or elimination from the thyroid gland. If the EHL is short and the duration from RIU to elimination is rapid, iodine is not stored in the thyroid tissue for a long duration. Therefore, patients with low CT values are likely to have a higher RIU rate for elimination in the thyroid gland.
In patients with Graves' disease, CT value decreases as the TRAb level increases, which may be attributed to the rate of iodine metabolism in the thyroid tissue. TRAb is a TSH receptor antibody that both inhibits and continuously stimulates the TSH receptors to oversecrete thyroid hormones. Thus, high TRAb levels result in low iodine storage in the thyroid tissue, potentially reflected by a low-density CT value. These results suggest that the lower CT value in patients with Graves' disease or Plummer disease is due to the faster rate of iodine metabolism. The results also suggest that the low CT value in patients with Graves' disease and Plummer disease is due to the high RIU of the thyroid gland and excessive thyroid hormone production, which results in a faster iodine efflux rate and, consequently, a lower iodine concentration in the thyroid gland.
The iodine concentration in the thyroid gland is strongly correlated with CT value; however, factors other than iodine concentration can also affect CT value. Imanishi et al. reported that a decrease in CT value indicates a decrease in the colloidal area associated with a decrease in iodine concentration within the colloid and an increase in the ratio of follicular cells and interstitial structures to the total thyroid gland [5]. Han et al. reported that CT value is correlated with the standardised uptake value on positron emission tomography images and that CT value is correlated with inflammation in the thyroid tissue [4]. The CT values of follicular cells and interstitial tissue are comparable with those of soft tissue, resulting in lower CT values than those in normal thyroid tissue. A lower CT value may also reflect an increase in the ratio of follicular cells and interstitial tissue to total thyroid tissue.
In addition, CT value is correlated with EHL and reflects the iodine metabolic capacity of the thyroid gland. EHL is an index required for determining the administered radioactivity of I-131 therapy. EHL should be measured at least twice within 8 days of administration of the I-131 test dose and repeated three or more times for accurate measurement. A fixed value is sometimes used to estimate the absorbed dose for I-131 therapy, as the measurement of the EHL is complicated. However, because calculating the absorbed dose with EHL as a fixed value can lead to errors, the EANM guidelines do not recommend determining radioactivity without considering thyroid volume and iodine kinetics [12]. Moreover, CT value could be used to estimate EHL and optimise the thyroid absorbed dose if EHL is not measured, as CT value is correlated with EHL. However, the extent to which the use of CT value improves the cure rate remains to be determined in future studies.
The present study has some limitations. First, the CT images of the patients were obtained before the initiation of treatment; however, the timing differed from that of the blood tests by approximately 1 week to 1 month. Changes, such as iodine restriction or the withdrawal of antithyroid medication, occurred in some cases during this period. However, no significant relationship with thyroid hormones was observed in the present study, as the thyroid hormones are sensitive to medication. Previous studies have reported that CT value is weakly correlated with TSH and FT4 [4, 6]. Therefore, the timing of CT imaging and blood sampling should be aligned to accurately determine the correlation between the thyroid hormone levels and CT value. Second, the CT value in patients with Plummer disease was calculated by including the normal areas of the thyroid gland. CT value calculated using this measurement method was correlated with EHL. However, as the number and size of nodules may affect the CT value in different cases, it is necessary to separate nodule areas from normal thyroid areas and calculate CT value to confirm their relationship with thyroid function. Finally, although the ROIs were set for the entire thyroid gland in the present study, some cases had low CT values and some CT values were almost equivalent to those of the surrounding soft tissues. Determining the thyroid contour in such cases was difficult, which may have contributed to errors in the calculation of CT value. As the ROIs were set by a single nuclear medicine specialist in the present study, inter-examiner errors were not examined. In addition, artefacts may have appeared owing to the influence of the clavicle and jaw, which may have changed the CT value. Such effects are considered to be one of the factors causing errors in the calculation of CT value.