The Change in 123I-Uptake Between 3-and 24-Hours is Useful in Predicting Early Response to Methimazole in Patients with Graves' Disease

Some patients with Graves' disease respond well to anti-thyroid drug treatment but others do not. Factors determining the patient's responsiveness to the medical treatment are unclear, but the intrathyroidal iodine pool is believed one of the important factors. In this study, we found that delta radioactive iodine uptake (RAIU) (RAIU at 24 h-RAIU at 3 h) is useful in predicting early response to treatment with methimazole (MMI). Among 32 patients with Graves' disease, who were given 30 mg MMI as an initial dose, 11 patients responded quickly to MMI-treatment. Within one month, serum free T4 levels decreased to below the normal range in 6 patients ( 125 pmol/L) to the normal range in 5 patients. When these rapid responders (group A) were compared with the remaining 21 patients who showed a more gradual response to MMI-treatment (group B), a different pattern of 123I-thyroid uptake was noted. RAIU at 3 h was significantly higher in group A than in group B, while RAIU at 24 h was similar in the two groups. As a result, rapid responders had a significantly lower delta RAIU value than gradual responders (-0.7 +/- 8.4% in group A, 14.2 +/- 8.2 in group B, P < 0.01). No significant difference was found between the two groups in various pre-treatment parameters such as severity and duration of thyrotoxicosis, the titer of TSH receptor antibodies (TRAb), frequency of positive antithyroglobulin antibodies (TGHA), urinary excretion of iodine and thyroid volume. The incidence of positive antithyroid microsomal antibodies (MCHA) was higher in group A than in group B, and thyroid ultrasonography showed a tendency to low echogenicity in group A. delta RAIU was negatively correlated with the reduction in the serum free T4 level during the first two weeks after MMI-treatment was initiated (r = -0.60, P < 0.01). Moreover, delta RAIU correlated positively with the biological half-life of the intrathyroidal iodine, calculated in a different series of 24 patients with Graves' disease who received radioisotope treatment (r = 0.54, P < 0.01). The low delta RAIU value is considered to reflect the rapid turnover of the intrathyroidal iodine, and may be related to the small intrathyroidal iodine pool. delta RAIU is useful in predicting early responsiveness of patients with Graves' disease to MMI-treatment.