Effect of Lugol’s solution on 131I therapy efficacy in Graves’ disease

Lugol’s solution could control thyroid function and suppress 131I uptake in hyperthyroidism. This study aimed to investigate the appropriate time to withdraw Lugol’s solution before 131I therapy (RIT) in Graves’ disease (GD) patients, and how this should influence 131I uptake and RIT outcome. Two groups (125 cases and 1805 cases) of GD patients received RIT, who were pre-treated with and without Lugol’s solution (RI-CI group and RI group). The RI-CI group was further divided into the following sub-groups depending on the duration span between Lugol’s solution withdrawal and RIT: sub-group A, 4–7 d (n = 49); sub-group B, 8–14 d (n = 41); and sub-group C, 15–30 d (n = 35). The highest radioactive iodine uptake rate (RAIUmax), effective half-life (Teff), TRAb, and free triiodothyronine (FT3) and free thyroxine (FT4) levels were compared, and therapeutic outcome was evaluated. There were no significant differences in RAIUmax, TRAb, and Teff among the four sub-groups (P > 0.05). Both FT3 and FT4 levels in sub-groups A and B were lower than those in group RI and sub-group C (P < 0.05). The outcome of non-hyperthyroidism (euthyroidism + hypothyroidism) in groups RI-CI and RI was significantly different at post-RIT month 1 and 3 (P < 0.05). However, intergroup differences at 6 and 12 months were not significant (P > 0.05). Withdrawal of Lugol’s solution 4–7 or 8–14 d before RIT does not influence 131I uptake and RIT efficacy in GD. Moreover, in order to avoid a rapid increase in thyroid hormone levels at the same time, Lugol’s solution should be withdrawn 4–7 d before RIT.


Background
The treatment of hyperthyroidism or Graves' disease (GD) by 131 I is a widely accepted approach in cases with preserved iodine uptake capacity of the thyroid gland [1,2]. However, in GD patients with aggravated symptoms, immediate 131 I therapy (RIT) may be a risk factor for hyperthyroidism crisis if pre-treatment is not administered.
Lugol's solution (containing 5% iodine and 10% potassium iodide) can quickly affect the iodine metabolism in the thyroid, decrease the iodine pool, prevent iodine uptake, inhibit proteolytic enzymes, reduce thyroglobulin decomposition, inhibit the synthesis and release of thyroid hormones, and quickly suppress serum thyroid hormone levels. Lugol's solution can also reduce blood flow to the thyroid gland, antagonize glandular congestion, and shrink and harden the gland. Clinically, Lugol's solution is often used with the aim of pre-operative preparation in GD patients. Lugol's solution is also used in patients with precursor hyperthyroidism crisis, hyperthyroidism patients with liver damage, neutropenia or other severe complications after antithyroid drug (ATD), and patients with aggravated symptoms after RIT. On the basis of our clinical experience, Lugol's solution pre-treatment should be taken for no more than 2 months. Lugol's solution tastes bitter and it is irritating to the gastrointestinal tract, so it is recommended to use a dropper on the bread slice or fully dilute it with a sweet drink such as apple juice.

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However, the use of Lugol's solution shall affect 131 I uptake and the efficacy of RIT in GD patients. Hence, the appropriate commencement and secession of Lugol's solution as a pre-treatment before RIT are an interesting and important issue to address [3][4][5][6][7]. This study aimed at investigating the appropriate time to withdraw Lugol's solution without affecting RIT efficacy in patients with GD, so as to guide its clinical application in such scenario.

Patients
This study was conducted between January 2014 and November 2019. Collectively, 1930 patients (510 male and 1420 female; age range 17-63 years) received RIT for the first time were enrolled. GD was diagnosed based on guideline recommendations [2]: increased thyroid hormone and decreased thyroid-stimulating hormone (TSH) levels; elevated radioactive iodine uptake (RAIU), diffuse goiter, exophthalmos, or pretibial myxedema; or increased thyrotropin receptor antibody (TRAb) levels. During the enrollment period, patients underwent a complete physical examination. The study protocol was approved by the Tianjin Medical University General Hospital Ethics Committee, and all patients provided written informed consent.
Among the 1930 patients, 125 cases were pre-treated with Lugol's solution (RI-CI group) and 1805 without (RI group). 7-14 days of iodine-consuming restriction and more than 3 days antithyroid medication (e.g., methimazole and propylthiouracil) secession required for the patients prior to RIT. The urinary iodine concentration (UIC) was measured whenever it was feasibly applied [2]. Patients in the RI-CI group were treated with Lugol's solution (0.5 ml tid) for at least 14 d in advance. Depending on the secession use (withdrawal durations) of Lugol's solution, RI-CI group was further divided into three sub-groups: sub-group A, 4-7 d (n = 49); sub-group B, 8-14 d (n = 41); and sub-group C, 15-30 d (n = 35). All patients were subjected to RIT and followed up for at least 12 months.
According to the Chinese guideline [2], if patients still have obvious symptoms after RIT, symptomatic treatment such as β-blockers should be given to relieve symptoms. In patients with severe hyperthyroidism or with obvious complications or comorbidities (such as severe heart failure, severe arrhythmia, and frequent hypokalemic periodic paralysis), ATD treatment should be resumed after RIT. The commencement of ATD treatment was 3-30 days after RIT, and the average treatment duration was 90 days (45, 150 days). In this study, a small percentage of patients with severe hyperthyroidism (251 cases in total, accounting for 13%, 13 cases out of 125 in RI-CI group and 238 cases out of 1805 in RI group) were treated with ATD to control hyperthyroidism after RIT. There was no statistical difference in percentage of the patients who were treated with ATD between the two groups (χ 2 = 0.802, P = 0.371).

Highest RAIU and effective half-life (T eff )
Radioactivity was measured using a Na 131 I nuclear multifunctional instrument (MN-6300XT Apparatus, Technological University, China). Counting was conducted for 1 min with the single-channel analyzer in the differential mode, and 74 kBq of 131 I was administered 24 h before the RAIU measurement. The thyroid radioactivity uptake was calculated using background radiation correction and a reference standard: thyroid uptake = (neck counts-the remainder of the body counts)/(reference counts-room counts). The RAIU of the thyroid was determined dynamically at 24, 48, and 72 h to determine the highest RAIU (RAIU max ) and T eff .

Measurement of thyroid volume
Thyroid volume was assessed by ultrasound (GE Logiq 400 Pro, GE Healthcare). The maximum width (W), depth (D), and length (L) were measured in both lobes of the thyroid gland by ultrasonography. Thyroid weight was calculated using the following formula: thyroid weight (g) = 0.479 × ( WR × DR × LR + WL × DL × LL) cm 3 [8].

Determination of RIT dose
Each patient was provided with information on the principles of the therapy and regulations of radioactivity protection. All patients provided written informed consents for the treatment protocol. The therapeutic activity (A) of 131 I was calculated using Marinelli's formula [9]: A = 0.67 × 100 (GY/g) × estimated thyroid weight (g)/RAIU max (%) × T eff (d). The estimated dose absorbed by thyroid tissue was 100 (GY/g); the correction factor was 0.67. In our hospital, 131 I was administered in rooms specially adapted and intended for the procedure.

Curative effect analysis
All patients were followed up for at least 12 months. The curative effect was classified, according to the Chinese guideline [2], as complete remission, partial remission, invalid or relapse, and hypothyroidism. Complete remission was defined as disappearance of hyperthyroidism signs and symptoms entirely, and normalization of serum FT 3 , FT 4 , and stimulated TSH (sTSH) levels. Partial remission was defined as alleviation of hyperthyroidism characterized by partial disappearance of the related signs and symptoms, and a significant decrease but not normalization of serum FT 3 and FT 4 levels. Invalid or relapse was defined as no improvement or even aggravation of signs and symptoms of hyperthyroidism, no decrease in serum FT 3 and FT 4 levels. Hypothyroidism was defined as the appearance of hypothyroidism signs and symptoms, lower than normal serum FT 3 and FT 4 levels, and higher than normal TSH level [2].

Statistical analysis
Normally distributed measurement data were expressed as the mean ± standard deviation ( x ± SD), and statistics was performed by using IBM SPSS 26.0 software (IBM Corporation, Armonk, NY). One-way analysis of variance was used to compare RAIU max , T eff , TRAb, and FT 3 and FT 4 levels before RIT among multiple groups or sub-groups. The least significant difference (LSD) test was used to compare two groups or sub-groups. The non-hyperthyroidism rates (euthyroidism + hypothyroidism) at four follow-up time points (1, 3, 6, and 12 months after RIT) were compared using chi-square test, with significance set at P < 0.05. The overall curative effect in the RI-CI and RI groups was compared using the relative to the identified distribution (ridit) analysis.

Baseline characteristics
Sex, age, disease duration, and thyroid weight did not differ significantly among the groups (Table 1). Radioactive 131 I dose per gram of thyroid tissue (IDPG) and UIC significantly differed among the groups (P = 0.035, 0.000). The IDPG in sub-groups A-C was significantly higher than that in the RI group (all P < 0.05). The normal range of measurement of UIC was 100-300 ug/L. The UIC in sub-groups A-B was higher than that in sub-group C and RI group.

Comparison of FT 3 , FT 4 , and TRAb levels and RAIU max and T eff
The results shown in Table 2 indicated no significant differences in the RAIU max , T eff , and TRAb status among the groups (P > 0.05). The FT 3 levels in sub-groups A and B were significantly lower than those in the RI group and subgroup C (all P < 0.05). The FT 4 values of sub-groups A and B were obviously lower than those in the RI group and subgroup C (all P < 0.01). Table 3 indicated different RIT responses in the RI-CI and RI groups at each point. The non-hyperthyroidism rates (euthyroid + hypothyroid) between the RI-CI and RI groups were statistically significant at 1 and 3 months after RIT (P < 0.05), but no significant differences were found at 6 and 12 months (P > 0.05). Thus, the application of the Lugol's solution would not affect the overall efficacy of RIT on GD ( Fig. 1 in detail). Table 4 displayed the distribution of efficacy in the patients. All patients were followed up for at least 12 months. Considering the RI group as a whole, the average R value of the ridit analysis for the RI-CI group was 0.52. The 95% confidence interval was 0.5, which meant that the difference between the two groups was not statistically significant.

Discussion
GD is an autoimmune thyroid disorder. The primary clinical manifestation of GD is the presence of TRAb and overproduction of thyroid hormone from the gland [10]. ATDs are used to reduce thyroid hormone production, but a relatively high relapse rate (50%) of GD has been reported [11]. Previous studies have recommended that if ATDs fail after 18 months, RIT or thyroidectomy should be applied [11,12]. The purpose of RIT is to destroy sufficient thyroid tissue by β-particles emitted from 131 I. Patients with significantly elevated thyroid hormone levels and obvious symptoms should receive pre-treatment before RIT. Moreover, some patients may show aggravation of the symptoms after withdrawal of ATD administration or have adverse complications. Pre-treatment is also necessary in the above scenarios. These preparations before RIT include administration of ATDs (e.g., methimazole and propylthiouracil), lithium carbonate, or prednisone [13][14][15][16][17][18][19][20]. Lugol's solution can be used in these circumstances as well as to prevent hyperthyroidism crisis.
Iodine consumption restriction (7-14 days) was required for the patients prior to RIT. The normal range of measurement of UIC was 100-300ug/L. The UIC results showed that the subjects of sub-group C and RI group succeeded in iodine restriction. The UIC results in sub-groups A-B were higher than that in sub-group C and RI group (in the normal UIC range). This was because the time between withdrawal of Lugol's solution and RIT was relatively short. However, although participants in sub-group A and B might not succeed in iodine restriction, this did not affect 131 I uptake in both two sub-groups. Our effort in this management of patents was due to the need to control the disease as well as ensure the safety of RIT.
Iodine plays an important role in thyroid physiology resulting from its importance as a requisite substrate for the synthesis of thyroid hormones and from its action as a regulator of thyroid function [21]. Lugol's solution was invented in 1829 by the French physician Jean Guillaume August Lugol, initially as a medicine for tuberculosis [22]. It is a solution of elemental iodine (5%) and potassium iodide (KI, 10%) together with distilled water. Lugol's solution causes an increased thyroidal iodide uptake and inhibits thyroid peroxidase which attenuates iodide oxidation and organification [23]. Iodine in supraphysiological doses inhibits thyroid hormone synthesis (defined as the Wolff-Chaikoff effect) [24] and reduces thyroid hormone release into the circulation [25]. Escaping from the acute Wolff-Chaikoff effect is associated with a decrease in thyroid sodium/iodide symporter activity, causing a decrease in intrathyroid iodide concentration [26]. In addition, the clearance of iodine by the thyroid gland of hyperthyroidism patients is altered and aggravation of hyperthyroidism after iodine is possible (defined as the Jod Basedow effect) [27]. Lugol's solution has for almost a century been used as an adjuvant treatment in patients with GD planning for thyroidectomy. Lugol's solution has effects in decreasing thyroid hormone levels, thyroid vascularity, intraoperative blood loss and in reducing blood flow within the thyroid gland during thyroidectomy [28][29][30][31][32][33]. However, reports on the use of Lugol's solution for the treatment of hyperthyroidism are limited. Nevertheless, there is no research on the topic of the duration of its usage, the ideal withdrawal or secession time, and its effects on RIT.
RAIU max is one of the important factors for calculating the therapeutic 131 I dose in GD patients [34][35][36]. The use of Lugol's solution may affect iodine metabolism in thyroid gland, which inevitably influence RAIU, resulting in an impaired therapeutic effect of 131 I. The discontinuation time of Lugol's solution is one of the key factors for the above process. Experts in the Chinese guideline [2] suggested that Lugol's solution should be discontinued for 2-3 weeks before RIT. However, the hyperthyroidism symptoms of some patients will worsen due to this withdrawal. Therefore, it is necessary to study the most appropriate secession time point for Lugol's solution which will not cause a rebounded excessive thyroid hormone levels, at the same time will restore the thyroid RAIU to allow RIT implementation with good outcomes. Our results indicated that from 4 to 14 days after discontinuation of Lugol's solution, neither significant effect on the RAIU max nor symptom exacerbation were found. In addition, preventing a rapid increase in the serum thyroid hormone level after RIT should be considered as well. Therefore, our study recommended that RIT should be performed 4-7 d after discontinuation of Lugol's solution pre-treatment.
The efficacy analysis showed that the application of Lugol's solution would not affect the overall efficacy. The non-hyperthyroidism rates (euthyroid + hypothyroid) in the RI-CI group were significantly lower than those in the RI group at 1 and 3 months after RIT, but no significant differences were found at 6 and 12 months between the 2 groups. These findings could be due to the iodine-escape effect during the first 3 months after the Lugol's solution withdrawal, which influenced the short-term effect. The hypothyroidism rates at 1 and 3 months were significantly lower than that of the RI group; however, the euthyroidism rates were not significantly different between the two groups. With the prolonged follow-up period, the non-hyperthyroidism rates (euthyroid + hypothyroid) at 6 months and 12 months were not significantly different from those of the RI group.
This study lasted for nearly 6 years and had a large sample size (1930 cases) with relatively complete data. However, there are still some limitations. First, the complex metabolism of iodine in human body is affected by various factors, but all individual differences were challenging to be evaluated. Second, the out-patient follow-up system records the results of the judgment made by the physicians based on the patient's complaints and physical examination results, while the lack of objective records such as heart rate and weight is also a defect. Third, the numbers of patients who use Lugol's solution need to be accumulated. Forth, the follow-up time was still very short. Therefore, the follow-up period should be extended and more comprehensive research should be conducted to validate our preliminary findings.

Conclusion
In summary, withdrawal of Lugol's solution for 4-14 d will not influence 131 I uptake and the efficacy of RIT in GD patients. Moreover, in order to avoid a rapid increase in thyroid hormone levels after RIT at the same time and ensure a safer peri-treatment period, we recommend that RIT should be performed after 4-7 d of withdrawal. In addition, it should be noted that 4-14 days of Lugol's solution withdrawal may be not enough for strict iodine restriction, and this circumstance was only applied for cases with short duration of Lugol's solution usage.