Effect of previous administration of potassium iodine and different durations of low iodine diets for radioiodine therapy on the treatment of Graves' disease in iodine-rich areas

Abstract

Purpose: To examine whether adherence to a low-iodine diet (LID) enhances the therapeutic efficacy of radioiodine therapy (RAI) in Graves' hyperthyroidism (GH) in iodine-rich areas.

Methods: We retrospectively evaluated 185 patients with GH from Aichi (n = 114) and Hokkaido (n = 71) Prefectures. Patients aged ≥ 18 years with GH who underwent RAI between December 2012 and March 2022 were divided into subgroups based on pretreatment with anti-thyroid drug (ATD) or potassium iodide (KI). Patients were followed up with LID from 18 days (group A) or 7 days (group H) before RAI to 3 days after RAI. The dose of radioactive iodine 131 (¹³¹I) was adjusted to deliver > 100 Gy to the thyroid. The associations between urinary iodine concentration on UIC2 vs. 24hRU and UIC2 vs. the 1-year RAI success rate (SR) were investigated.

Results: Compared with UIC1, UIC2 was significantly decreased in all subgroups (P < 0.01). An inverse correlation between UIC2 and 24hRU was observed in the four groups; however, the difference was insignificant. The SR in groups A and H was 85% and 89%, respectively. Univariate analysis revealed no association between UIC2 and SR in each group. Additionally, stratification of the 185 patients into quartiles using UIC2 yielded no significant differences in SR (p = 0.79).

Conclusions: LID sufficiently reduced UIC in patients undergoing RAI. Although a lower UIC2 may increase 24hRU, it did not increase the success of RAI. The benefit of LID in enhancing the efficacy of RAI in GH treatment remains uncertain.

Keywords: Graves’ hyperthyroidism; Iodine-rich areas; Low iodine diet; Radioiodine therapy; Urinary iodine concentration.

Fig. 1

Protocol for radioactive iodine treatment at the two facilities. Aichi group: Patients in Nagoya University Hospital living in Aichi. ATD, antithyroid drug; FT3: free triiodothyronine; FT4: free thyroxine; Hokkaido Group: Hokko Memorial Hospital, Hokkaido; KI: potassium iodine; p.o.: per os; RU: radioiodine uptake; TRAb: thyrotropin receptor antibody; UIC: urinary iodine concentration

Fig. 2

Relationship between 24 h radioiodine uptake (24hRU) and urinary iodine concentration on the day of treatment. Group A1: Patients in Nagoya University Hospital living in Aichi pretreated with an antithyroid drug (ATD) alone. Group A2: Patients in Nagoya University Hospital living in Aichi pretreated with ATD and potassium iodide (KI) or KI alone. Group H1: Patients in Hokko Memorial Hospital living in Hokkaido pretreated with ATD alone. Group H2: Patients in Hokko Memorial Hospital living in Hokkaido pretreated with ATD and KI, or KI alone. Spearman’s rank correlation test is used

Fig. 3

24hRU in patient groups stratified according to UIC2 quartiles *: p = 0.008; **: p = 0.005. 24hRU: 24 h radioiodine uptake. UIC2: urinary iodine concentration measured on the day of radioactive iodine 131I therapy. The Kruskal-Wallis test is used

Fig. 4

Association between UIC2 and effective half-life of ¹³¹I in the Aichi group. Group A1: Patients in Nagoya University Hospital living in Aichi pretreated with ATD alone. Group A2: Patients in Nagoya University Hospital living in Aichi pretreated with ATD and KI or KI alone. Spearman’s rank correlation test is used. ATD, antithyroid drug; KI, potassium iodide; UIC, urinary iodine concentration

Fig. 5

Average T1/2eff among the four patient groups stratified by UIC2 into quartiles. Avg. = average value. The Kruskal–Wallis test is used

Fig. 6

UIC2 according to RAI evaluation in groups A and H. Group A: Patients living in Aichi, Nagoya University Hospital. Group H: Patients in the Hokko Memorial Hospital living in Hokkaido. UIC2, urinary iodine concentration measured on the day of radioactive iodine I-131 µg/gCRE therapy. Success: RAI was considered successful when euthyroidism, subclinical hypothyroidism, or hypothyroidism was achieved within 1 year after RAI. Failure: Other than success. The Kruskal–Wallis test is used. Avg. = average value