Empiric Radioiodine for Hyperthyroidism

Outcomes, Prescribing Patterns, and Its Place in the Modern Era of Theranostics

Emma Boehm; Yung Hsiang Kao; Jeffrey Lai; Paul R. Wraight; Dinesh A. Sivaratnam


Clin Endocrinol. 2022;97(1):124-129. 

In This Article


There were 229 patients with hyperthyroidism who were treated with I-131 between January 2016 and June 2021. Eighty-three patients were excluded due to incomplete follow-up data. For the 146 patients included in the study the mean age was 57 years old (range 21–87) and 73% were female. In terms of the aetiology of hyperthyroidism; 61 (41.8%) had Graves' disease, 37 (25.3%) had toxic multinodular goiter, and 48 (32.9%) had toxic adenoma. The mean follow-up time for any outcome was 13.6 months (range 0.9–60.5), with the mean follow-up for remission of 13.3 months (range 0.9–60.5) and persistence of 14.5 months (range 4.4–45.5).

Across the entire cohort, 118/146 (80.8%) of patients had remission of hyperthyroidism following I-131 therapy. Table 1 outlines patient demographics and clinical factors stratified for persistence or remission of hyperthyroidism. On univariate analysis for the subpopulation with available data, higher free T4 and lower thyroid-stimulating hormone were associated with persistence of hyperthyroidism (p = .048 and <.001, respectively). There was no difference in patient age, sex, anti-thyroid drug (ATD) treatment dose or duration of ATD cessation between the remission and persistence groups. Only 1/146 (0.7%) had documentation of worsening symptomatic thyrotoxicosis after I-131 therapy requiring emergency care. This patient was markedly thyrotoxic (fT3 > 46.1 pmol/L) at the time of treatment, and fT4 increased from 27.3 to 44.8 pmol/L four days after I-131 was given. Hypothyroidism occurred in 58/146 patients (39.7%). Outcomes are summarized in Table 2, stratified by disease aetiology, 20-min Tc-99m pertechnetate uptake and prescribed activity.

Graves' Disease

Sixty-one (73.8%) patients with Graves' disease achieved remission from hyperthyroidism following I-131 therapy. Of these, 40 (65.6%) were rendered hypothyroid at a mean time of 7.7 months (range 0.9–56.2). Mean prescribed activity for remission was 375 ± 83.9MBq (10.1 ± 2.3 mCi) whereas in persistence of hyperthyroidism it was lower at 325 ± 73.0MBq (8.8 ± 2.0 mCi); the difference was statistically significant (p = .035).

There was a trend towards patients with higher Tc-99m pertechnetate uptake remaining hyperthyroid post I-131 therapy, although this did not reach statistical significance in either a univariate (p = .059) or a multivariate analysis (p = .055). Further analysis revealed a weak inverse correlation between Tc-99m pertechnetate uptake and the prescribed I-131 (Pearson's correlation coefficient −0.33; p = .009) (Figure 1). Characteristics of five Graves' disease patients with very high Tc-99m pertechnetate uptake, >15%, are summarized in Table 3.

Figure 1.

Prescribed I-131 activity versus 20-min Tc-99m pertechnetate uptake percentage in Graves' disease.

For dosimetric sub-analysis, meta-analyses of published studies showed the average Graves' disease thyroid mass to be 36 g and I-131 Residence Time to be 7.67 days (Supporting Information Data). The corresponding value of Ē is 2.837 Gy.g per MBq.day.[4] Applying these parameters into Equation 1, our Graves' disease mean absorbed dose was estimated to be 225 ± 50 Gy for the remission group vs 195 ± 44 Gy for the group with persistent hyperthyroidism.

Toxic Nodule

There was a 90.1% remission rate for patients treated for a toxic nodule. No significant difference was found between remission versus persistence of hyperthyroidism in terms of Tc-99m pertechnetate uptake or prescribed activity.

Toxic Multinodular Goitre

Twenty-eight (75.7%) patients with toxic multinodular goitre achieved remission from hyperthyroidism after I-131 therapy. There was no significant difference in the prescribed activity between remission vs persistence of hyperthyroidism groups, 475.7 ± 57.5MBq (12.9 ± 1.6 mCi) vs 460 ± 53.5MBq (12.4 ± 1.4 mCi) (p = .463). Tc-99m pertechnetate uptake was generally higher in patients who had persistence of hyperthyroidism (7.3 ± 3.9%) compared to those who achieved remission (2.8 ± 3.0%) in both univariate (p = .014) and multivariate analysis (p = .009).