Patterns of Thyroid Radioiodine Uptake: Jordanian Experience

MATERIALS AND METHODS

RESULTS

The characteristics of the patient groups are summarized in Table 1. Generally, thyroid disorders are more common in women than men, and most of the patients are middle-aged. The mean uptake values for the euthyroid group, toxic-nodule group, and Hashimoto's thyroiditis group were 15% ± 7%, 19% ± 9%, and 19% ± 15%, respectively. There was no significant statistical difference between mean uptake values for these 3 groups (P > 0.5). However, both the euthyroid and the toxic-nodule groups had a narrow 95% confidence interval (CI), whereas the Hashimoto's thyroiditis group had a wide 95% CI (Fig. 1). The mean uptake value for the subacute thyroiditis group was 3% ± 4%, with a narrow 95% CI, and was statistically different from all other groups (P < 0.05) (Fig. 1). The mean uptake value for Graves' disease was 40% ± 14%, with a narrow 95% CI, and was statistically different from all other groups (P < 0.05) (Fig. 1). Scatter of the actual radioiodine uptake values for all subjects is shown in Figure 2.

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FIGURE 1. 

Ranges (95% CI) and mean values of radioiodine uptake in Jordanian patient groups.

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FIGURE 2. 

Scatter of actual radioiodine uptake values for Jordanian patient groups.

Four euthyroid patients had extremely low radioiodine uptake (<5%). Two of those 4 patients (2% and 3% uptake) underwent contrast CT 1 d before uptake measurement. One also had too many cold nodules as part of a multinodular goiter. The third patient (4% uptake) was in early recovery from recent subacute thyroiditis. Regarding the fourth patient (4% uptake), we failed to find a cause for his low uptake value.

DISCUSSION

Thyroid scintiscans and radioiodine uptake measurement are performed more commonly in Europe than in the United States (2). This test is used mainly to identify the cause of thyrotoxicosis (1). In Jordan, this test is widely used for assessment of thyroid disorders, including thyrotoxicosis, nodule characterization, and Hashimoto's thyroiditis. Although ¹²³I has better spatial resolution and less radioactive burden to the patient than ¹³¹I, ¹²³I is not used in Jordan or, probably, in many other developing countries because as a cyclotron product its availability is limited (13). Instead, ¹³¹I is widely used in Jordan and in many other developing countries for thyroid scintiscans and radioiodine uptake measurement because its long half-life (8.1 d) and low price make it widely available in most nuclear medicine laboratories in this region of the world. Dosing for thyroid scintigraphy in our laboratory is within the range recommended by the Society of Nuclear Medicine procedure guideline (14).

Although use of a neck phantom is recommended during radioiodine uptake measurement (15), a phantom was not used in our laboratory. Chervu et al. (16) studied radioiodine uptake measurement with and without a neck phantom. They found that a neck phantom made the Compton scatter peak more prominent and closer to the total absorption peak for both ¹²³I and ¹³¹I. In addition, this finding was more obvious with ¹²³I than with ¹³¹I, because ¹²³I has lower photon energy. The use of a wide window setting, particularly with ¹²³I, would result in the inclusion of a significant amount of scatter. Moreover, they reported that 8 patients underwent radioiodine uptake measurement 3 times using 3 different phantoms and the radioiodine uptake values were different in the 3 measurements. They recommended that the neck phantom should be standardized in terms of the depth of capsule housing and the resulting attenuation factor.

In Jordan, nuclear physicians frequently are asked by clinicians what the reference range of radioiodine uptake is. There is no reported reference range for radioiodine uptake in this region of the world. It is well known that there is geographic variation in stable iodine intake, which significantly affects the body iodine pool and the value of radioiodine uptake (3). Radioiodine uptake is lower in a region with an adequate amount of iodine in the food than in an iodine-deficient region (4). In fact, the geographic variation and the possibility of changing dietary iodine intake in healthy persons point to the necessity of current and local determinations of the “reference range” of the thyroidal uptake of radioiodine if the results of this thyroid function test are to be properly interpreted (5). For this purpose, we reviewed the results of thyroid scintiscans and radioiodine uptake measurement performed in northern Jordan from 2004 to 2008. The mean thyroid radioiodine uptake in euthyroid subjects in our laboratory was 15% ± 7%, with a narrow 95% CI (13%−16%). Our euthyroid mean is comparable to means reported in the 1970s in North America (6–11) and in later studies using ¹³¹I in Europe (12). But our euthyroid mean is smaller than the mean reported in one study that used ¹²³I in the early 1990s in Boston (17).

The nuclear medicine laboratory at King Abdullah University Hospital is the only nuclear medicine facility in northern Jordan at this time. Patients referred to us are coming from urban regions, rural regions, refugee camps, mountainous regions, and desert. Regardless of this variation in the geographic regions of our population within the northern Jordanian districts, we think that our population has sufficient stable iodine intake for 2 reasons: like developed countries, Jordan adds a stable iodine supplement to table salt, and Jordan and other middle east countries have a high consumption of iodine-rich food, including bread, dairy products, sea food, eggs, and vitamin supplements (6,10,11,17). Our euthyroid subjects were not selected from a healthy population. They were patients referred for investigation of a potential thyroid disorder. However, we believe that this is a realistic means of estimating thyroid radioiodine uptake in euthyroid subjects in this region.

The mean thyroid radioiodine uptake in our patients with Graves' disease was 40% ± 14% with a narrow 95% CI (37%−43%). As noted in other series (4), there is virtually no overlap between uptake ranges in euthyroid groups and Graves' disease groups. The mean thyroid radioiodine uptake in the subacute thyroiditis group was 3% ± 4%, with a narrow 95% CI (0%−4%). Our data showed no significant overlap between the range of radioiodine uptake in the euthyroid group and the subacute thyroiditis group in northern Jordan, as has also been noted in other series (18). It appears that radioiodine uptake testing discriminated between Graves' disease and subacute thyroiditis in our population. However, although no significant overlap exists between the uptake ranges of the Graves', subacute thyroiditis, and euthyroid groups, there is some overlap in actual measured uptake values between these groups as noted in many other nuclear medicine laboratories. In toxic nodular hyperthyroidism, however, this may not be the case because the mean thyroid radioiodine uptake was 19% ± 9% and the range (95% CI of 15%−22%) overlapped both the euthyroid group range and the Hashimoto's thyroiditis group range. But when combined with image features, the diagnosis was easily established.

The mean thyroid radioiodine uptake in the Hashimoto's thyroiditis group was 19% ± 15%, with a wide 95% CI (12%−26%). The Hashimoto's thyroiditis group mean was slightly higher than the euthyroid mean. The Hashimoto's thyroiditis group range overlapped both the euthyroid group range and the toxic-nodule group range. It has been reported that Hashimoto's thyroiditis may cause high or low uptake (13,19). One study showed that the most common finding in Hashimoto's thyroiditis is an enlarged gland with diffusely increased uptake, a pattern identical to that found in Graves' disease (20). The high avidity for radioiodine reflects chronic stimulation of a failing thyroid gland by thyroid-stimulating hormone in an attempt to produce sufficient thyroid hormone for the body's needs (20). The response of thyroid follicular cells to endogenous thyroid-stimulating hormone is preserved in thyroiditis patients, and sodium iodide symporter function is suppressed by certain cytokines (13). In addition, it was found that Hashimoto's thyroiditis affects the intrathyroidal organification of iodine and is one of the most common causes of positive results on perchlorate discharge testing (13,20–22).

In this connection, we admit that several patients in our laboratory underwent ¹³¹I thyroid scintiscans and radioiodine uptake measurement with no clear indication for these tests and that, consequently, there was no realistic justification for the subsequent high thyroid radioactive burden in those patients. The main reason for such misuse of this test in Jordan is the unawareness of referring physicians (mostly primary care physicians and general surgeons) about the updated indications for this test. From the nuclear medicine side, we could not cancel the test because cancellation likely will be misunderstood from both the patient side and the referring physician side. However, this type of practice has changed over the last few years in Jordan and referrals have become more realistic. Now we see fewer referrals for euthyroid patients and Hashimoto's thyroiditis patients unless there is a nodule.

CONCLUSION

The reference range for 24-h radioiodine uptake in Jordan is comparable to that reported in North America and Europe because Jordanians likely have a sufficient daily intake of stable iodine. According to our experience in Jordan, radioiodine uptake discriminates between euthyroid patients, subacute thyroiditis patients, and Graves' disease patients. Radioiodine uptake in toxic-nodule patients was diagnostic only if combined with image findings. Mean radioiodine uptake in Hashimoto's thyroiditis was higher than the euthyroid mean, but its range was wide and overlapped with the euthyroid and toxic-nodule ranges.