Efficacy of radioactive iodine treatment of graves' hyperthyroidism using a single calculated 131I dose

Clinical factors influencing the success rate of radioiodine treatment for Graves' disease

AIMS

To investigate the impact of various clinical factors associated with Graves' disease on the success rate of radioiodine (RAI) therapy for Graves' disease within 3 years, and to determine the optimal range of iodine dosage per unit volume that yields the highest cure rate for Graves' disease within 1 year.

MATERIALS AND METHODS

This retrospective study included patients diagnosed with Graves' disease who underwent RAI therapy at the Second Affiliated Hospital of Anhui Medical University between October 2012 and October 2022. The cumulative success rate was analysed using the Kaplan-Meier method. Univariate and multivariate Cox proportional hazards regression models were employed to evaluate factors associated with successful treatment of Graves' disease. Outcomes were categorized as either success or failure for all patients.

RESULTS

Overall, 1994 patients were enrolled in this study, including 594 (29.8%) male and 1399 (70.2%) female patients. The success and failure groups comprised 1645 (82.4%) and 349 patients (17.6%), respectively, after a 3-year follow-up period. Multivariate regression analysis demonstrated that sex, antithyroid drug (ATD) use before RAI therapy, age, thyroid receptor antibody (TRAb) levels, iodine dose, thyroid mass, and early ATD use before RAI therapy were independent influencing factors for Graves' disease cure.

CONCLUSIONS

We found that female patients and those with TRAbs ≥31.83 IU/L and thyroid mass ≥ 73.42 g had a lower cure rate. Therefore, thyroid size, disease severity, and duration of disease should be comprehensively considered when making treatment decisions and iodine dose selection in clinical practice.

Efficacy of Anti-Thyroid Medications in Patients with Graves' Disease

INTRODUCTION

Graves' disease (GD) is an autoimmune disorder characterized by hyperthyroidism due to increased thyroid-stimulating hormone receptor antibodies (TRAb).The treatment of GD often consists of radioactive iodine therapy, anti-thyroid drugs (ATD), or thyroidectomy. Since few studies have collected data on remission rates after treatment with ATD in Saudi Arabia, our study aimed to assess the efficacy and the clinical predictors of GD long-term remission with ATD use.

METHOD

We conducted a retrospective chart review study of 189 patients with GD treated with ATD between July 2015 and December 2022 at the endocrine clinics in King Abdulaziz Medical City in Riyadh. All GD patients, adults, and adolescents aged 14 years and older who were treated with ATD during the study period and had at least 18 months of follow-up were included in the study. Patients with insufficient follow-up and those who underwent radioactive iodine (RAI) therapy or thyroidectomy as first-line therapy for GD were excluded from the study.

RESULTS

The study sample consisted of 189 patients, 72% of whom were female. The patients' median age was 38years (33, 49). A total of 103 patients (54.5%) achieved remission. The median follow-up period for the patients was 22.0 months (9, 36). Patients who achieved remission had lower mean free T4 levels (25.8pmol/l ± 8.93 versus 28.8pmol/l ± 10.82) (P value = 0.038) and lower median TRAb titer (5.1IU/l (2.9, 10.7)) versus (10.5IU/l (4.2, 22.5)) (P value = 0.001) than patients who did not achieve remission. Thirty-five out of 103 patients who achieved remission (34%) relapsed after ATD discontinuation. The patients who relapsed showed higher median thyroid uptake on 99mTc-pertechnetate scan than patients who did not relapse: 10.3% (5.19, 16.81) versus 6.0% (3.09, 12.38), with a P value of 0.03. They also received ATD for a longer period, 40.0 months (29.00, 58.00) versus 25.0 months (19.00, 32.50), with a P value of < 0.0001.

CONCLUSION

The remission of GD was achieved in approximately half of the patients treated with ATD; however, approximately one-third of them relapsed. Lower Free T4 and TRAb levels at diagnosis were associated with remission. Longer ATD use and higher thyroid uptake upon diagnosis were associated with relapse after ATD discontinuation. Future studies are necessary to ascertain the predictors of ATD success in patients with GD.

A retrospective study with long term follow-up of Graves' disease patients treated with low activities of 131Iodine

BACKGROUND

Previous studies have shown that application of relatively low 131I-Iodine activities can successfully be used to treat patients with Graves' disease (GD). We assessed treatment outcome in the long-term follow-up of our GD patients and influencing factors.

METHODS

We evaluated 521 GD patients in this retrospective clinical single-center study. In all patients we performed scintigraphy and thyroid uptake measurement after 4 and 24 hours using 10 MBq 123I and calculated administered activity using Marinellis' formula. Treatment was done according to national regulations. Minimal routine clinical evaluation of all patients was available after 6 weeks and after 3, 6 and 12 months. Success of treatment was defined as euthyroid state or hypothyroidism 6 months after therapy.

RESULTS

We usually applied relatively low 131I activities. Three hundred seven patients (58.9%) became hypothyroid within 21 years of follow-up. One hundred thirty-nine patients (26.7%) became euthyroid and stayed euthyroid until the end of follow-up. We found a plateau after 7 years of initial therapy with only a few patients becoming hypothyroid after that time and identified 75 patients (14.4%) with persistent hyperthyroidism or recurrence.

CONCLUSIONS

Treatment with relatively low 131I activities produce favorable responses as shown in previous works. We found a high proportion of patients with long-term euthyroid state. Application of low activities reduces radiation burden of patients and, depending on radiation protection legislation, may lead to shortened hospital stay and reduced costs. Therefore, we feel that application of higher activities to treat GD patients as recommended in several current guidelines should be reconsidered.

Outcomes of Radioactive Iodine Versus Surgery for the Treatment of Graves’ Disease: a Systematic Review and Meta-analysis

Graves’ disease (GD) is a systemic autoimmune disorder mainly affecting the thyroid gland. During GD management, the principal target is to control the hyperthyroid state. There have been three rather similarly effective modalities: medical therapy with antithyroid drugs (ATD), radioactive iodine (RAI), or surgical excision of the thyroid tissue (thyroidectomy). Defining the relative risks and benefits of each of the two potential definitive treatment options (RAI or thyroidectomy) is crucial for creating evidence-based therapy algorithms. This systematic review and meta-analysis aimed to compare the outcomes of these two treatment options. This is a systematic review and meta-analysis that analyzed the studies comparing RAI and thyroidectomy to treat GD. Studies were obtained by searching on Scopus, the Cochrane Central Register of Controlled Trials, and PubMed central database. The surgically treated group showed significantly lower failure rates, non-significantly lower cardiovascular morbidities, non-significantly higher complication rates, and significantly lower mortality rates. The RAI-related complications were mostly the development or worsening of Graves’ ophthalmopathy. This review and meta-analysis comparing surgery and radioactive iodine for the treatment of Grave’s disease from 16 well-conducted trials has shown that although surgery viz., total thyroidectomy was less frequently utilized for the treatment of Grave’s disease, it controlled the symptoms with greater success and without any worsening of Grave’s ophthalmopathy.

Patient outcomes following thyroid surgery for thyrotoxicosis

OBJECTIVE

Total thyroidectomy can be used as a definitive treatment modality for thyrotoxicosis. This study assessed the outcomes of patients treated with surgery at a single secondary care site.

METHOD

A retrospective cohort study was conducted analysing consecutive patients who underwent thyroid surgery for thyrotoxicosis between 24 November 2000 and 26 April 2019 (n = 595).

RESULTS

Total thyroidectomy was performed in 95.4 per cent of patients. Two-thirds of patients had Graves' disease histology. Of patients, 22.8 per cent became transiently hypothyroid whilst on levothyroxine (thyroid hormone replacement therapy). Transient and persistent hypocalcaemia was present in 23.3 per cent and 2.8 per cent of patients respectively. Recurrent laryngeal nerve palsy was transient and persistent in 3.6 per cent and 0.3 per cent respectively. Of patients, 2.5 per cent developed post-operative haematomas that required surgical evacuation in the operating theatre.

CONCLUSION

The overall complication rate for thyroid surgery is higher in thyrotoxic than in euthyroid patients. Compared to other treatment modalities, total thyroidectomy appears to be the most effective, definitive means of managing Graves' disease.

Predictors of Prolonged Euthyroidism After Radioactive Iodine Treatment for Graves' Disease: A Pilot Study

OBJECTIVE

Patients with Graves' disease who remain hyperthyroid under the treatment of antithyroid drugs (ATD) or cannot tolerate ATD usually receive radioactive iodine (RAI) to control disease activity. This pilot study aimed to identify predictors of prolonged euthyroidism > 12 months after receiving RAI.

METHODS

Demographic, clinical, and laboratory data from 117 patients receiving RAI were retrospectively collected, including age, gender, body surface area, smoking status, free thyroxine, thyrotropin, thyrotropin binding inhibiting immunoglobulin, microsomal antibody, thyroglobulin antibody, medication history, and thyroid volume. Only 85 patients without missing values were included in statistical analysis. The calculated RAI dose was the estimated thyroid volume × 0.4. The difference and ratio between the actual and calculated RAI doses were examined. A stepwise logistic regression analysis was conducted to identify important predictors of prolonged euthyroidism > 12 months. The cut-off values for discretizing continuous covariates were estimated by fitting generalized additive models.

RESULTS

Among the 85 patients on RAI, 18 (21.2%) achieved prolonged euthyroidism > 12 months, 38 (44.7%) remained hyperthyroid with decreased ATD doses, but 29 (34.1%) suffered permanent hypothyroidism and needed long-term levothyroxine. Logistic regression analysis revealed that patients with age > 66 years, 33 < age ≤ 66 years, quitting smoking vs nonsmoking or current smoking, 600 < micorsomal antibody ≤ 1729 IU/mL, 47% < thyrotropin binding inhibiting immunoglobulin ≤ 81%, 7 < thyroglobulin antibody ≤ 162 IU/mL, 0.63 < ratio between actual and calculated RAI doses ≤ 1.96, or taking hydroxychloroquine would have a higher chance of reaching prolonged euthyroidism > 12 months after receiving RAI. Its area under the Receiver Operating Characteristic (ROC) curve was 0.932.

CONCLUSION

Patients with Graves' disease who received an actual RAI dose close to the calculated RAI dose achieved prolonged euthyroidism > 12 months if they also took hydroxychloroquine during RAI treatment.

An Interesting Case of Thyrotoxicosis

Ectopic thyroid tissue can be present in the embryonal path of descent of the thyroglossal duct anywhere from the foramen caecum to the thyroid gland. However, for such ectopic thyroid tissue to be hyperfunctioning is quite rare. Here, we discuss a 56-year-old female patient who presented with persistent thyrotoxicosis for over 7 years. She had undergone thyroidectomy in 1982 for thyrotoxicosis and was rendered hypothyroid (thyroid-stimulating hormone of 75 μIU/mL). Whole-body technetium scan was done twice which did not show any uptake in the neck or other parts of the body and an empirical dose of 15 mCi of radioiodine therapy was also given to treat the thyrotoxicosis. She continued to be thyrotoxic and was on carbimazole 30 mg/day along with beta-blockers. In 2021, an Iodine131 whole-body scan revealed small remnant thyroid tissue and an ectopic thyroid tissue in a thyroglossal cyst. In such cases of persistent or recurrent thyrotoxicosis despite standard treatments, an ectopic location should be sought after and treated.

[Value of thyroid 99mTcO4- imaging ROI ratio for estimating 131I dose in individualized treatment of hyperthyroidism]

OBJECTIVE

To investigate the feasibility of using thyroid ^99mTcO₄^- imaging ROI ratio instead of 24 h radioactive iodine uptake (RAIU) for estimating ¹³¹I dose in individualized treatment of hyperthyroidism.

METHODS

We retrospectively analyzed the clinical data of 132 patients receiving ¹³¹I treatment in our department between January and June, 2019. According to their 3 h/24 h RAIU peak ratio, the patients were divided into peak forward (≥80%) group and no peak forward (< 80%) group. In the former group, the therapeutic ¹³¹I dose was calculated based the Marinelli formula (¹³¹I dose=thyroid mass×planned amount/24 h RAIU), and in the latter group, the correlation between the ROI ratio and the 24 h RAIU was analyzed, and the ¹³¹I dose was calculated using a modified Marinelli formula where 24 h RAIU was replaced by a converted ROI ratio. The two groups of patients were compared for antithyroid drug type and discontinuation time, thyroid hormones and related antibodies, thyroid area, thyroid mass and ¹³¹I dose. All the patients were and followed up for one year to analyze the treatment efficacy. The ROI ratios after the treatment were analyzed in the two groups using ROC curves.

RESULTS

There was a significant positive correlation between the ROI ratio and 24 h RAUI in the no peak forward group (Y=58.13 + 0.2X, R²=0.118, P < 0.05), and the formula for calculating ¹³¹I dose was converted into: ¹³¹I dose=thyroid mass× planned amount/(58.13+0.2×ROI ratio)%. Before the treatment, therapeutic ¹³¹I dose, thyroid hormone levels, TRAb, 3 h and 24 h RAIU, thyroid area, thyroid mass, and ROI ratio all differed significantly between the two groups (P < 0.05). At 3 months after treatment, thyroid hormone levels, TRAb, TPOAb, thyroid area, thyroid mass, ROI ratio, response rate, hypothyroidism rate, cure rate, remission rate, and nonresponse rate were similar between two groups (P>0.05). At the 1-year follow-up, the composition ratios of hyperthyroidism, hypothyroidism and cured cases remained similar between two groups (P>0.05). ROC curve analysis showed that at 3 months after treatment, the optimal cutoff values of ROI ratio for predicting hyperthyroid recurrence and hypothyroidism were 15.79 and 6.33, respectively.

CONCLUSION

Thyroid ^99mTcO₄^- imaging ROI ratio can be used for calculating ¹³¹I dose in individualized treatment of hyperthyroidism and for prognostic evaluation of the patients.

Relationship between body mass index and external exposure in hyperthyroid patients treated with iodine-131

We performed this study to evaluate the correlation between Body Mass Index (BMI) and Exposure Rate (ER) of hyperthyroid patients treated with iodine-131 so that in case of any relationship, we can improve the prescribed dose for the treatment of hyperthyroid patients and its side effects on the body and the environment. In this analytical and cross-sectional study, 30 patients with hyperthyroidism treated with liquid iodine-131 were randomly selected. We recorded demographic indicators (age, height, and weight) and ER. Patients were treated with the activity of 8-29 mCi (mean 14.4 mCi) oral I-131. The external radiation of patients was measured from a distance of one meter parallel to the patient's thyroid gland at intervals of 6 to 24 hours post-iodine-131 administration. ER measurements and other acquired data were statistically analyzed by R software and its methods. Data were normalized using the Shapiro method, and due to the non-normality of the data in the correlation test, the Spearman method was used. The measurements of this study represent two main findings: 1. There is a significant relationship between ER and iodine-131 activity because the observed significance level (P-value =0.002) is smaller than the predicted error value (0.01). 2. There is no significant relationship between ER and BMI because the observed significance level (P-value =0.082) is greater than the predicted error value (0.05). The present study's findings show a negative relationship between BMI and ER. Still, since the P-value is more than 0.05, this relationship is not statistically significant.

Clinical Outcomes of Repeated Radioactive Iodine Therapy for Graves' Disease

BACKGRUOUND

Radioactive iodine (RAI) therapy is a successful therapeutic modality for Graves' disease. However, RAI therapy can fail, and RAI therapy after antithyroid drugs (ATDs) has a lower remission rate. Therefore, many patients require repeated RAI therapy. This study investigated the clinical outcomes of repeated RAI therapy for Graves' disease.

METHODS

Patients who underwent RAI therapy as second-line therapy after failure of ATD treatment between 2001 and 2015 were reviewed. Remission was defined as hypothyroid or euthyroid status without ATD, and with or without levothyroxine at 12 months after RAI therapy.

RESULTS

The 1-year remission rate after 2nd RAI therapy (66%, 152/230) is significantly higher than that after 1st RAI therapy (48%, 393/815) or long-term ATD treatment after 1st RAI therapy failure (42%). The clinical response to 2nd RAI therapy was more rapid. The median time intervals from the 2nd RAI therapy to ATD discontinuation (1.3 months) and to the start of levothyroxine replacement (2.5 months) were significantly shorter than those for the 1st RAI therapy. A smaller goiter size, a longer time interval between the 1st and 2nd RAI therapies, and a longer ATD discontinuation period predicted remission after the 2nd RAI therapy. Finally, in 78 patients who failed the 2nd RAI therapy, the mean ATD dosage significantly reduced 5.1 mg over 12 months.

CONCLUSION

Repeated RAI therapy can be a good therapeutic option, especially in patients with smaller goiters and those who are more responsive to the 1st RAI therapy.

Efficiency of marketable decontamination agent and graphene oxide on 99mTc and 131I spillages in nuclear medicine department

Dealing with open sources of radioactive substances in nuclear medicine is a daily task since contamination due to radioactive spills may happen frequently. Proper and safe decontamination management is a vital procedure. However, regular purchase of decontamination agents incurs high costs and might be toxic due to their chemical properties. The purpose of this study is to compare graphene oxide, which is an environmentally friendly carbon-based material and marketable decontamination agent, in decontaminating radioactive spillage. Samples of pure 99mTc and 131I from the laboratory were spilled on a petri dish. The spill was immediately decontaminated with a marketable decontamination agent swab and varying concentrations of graphene oxide swab. The initial radioactivity of each swab containing 99mTc and 131I was measured using a dose calibrator. The absorbance spectra of each sample were analysed using an ultraviolet-visible spectrophotometer. The morphology image of graphene oxide was observed under field emission scanning electron microscope. For decontamination using a marketable decontamination agent, the radioactivity of 131I was slightly higher, whereas that of 99mTc was slightly lower than the high concentration of graphene oxide. The absorbance spectra of 99mTc and 131I that had been decontaminated using graphene oxide were observed at a range of 200 nm to 250 nm due ???* to the transition.

Clinical efficacy of fixed-dose radioactive iodine for the treatment of hyperthyroidism at a single centre: our experience

BACKGROUND

Iodine-131 is an effective treatment for thyrotoxicosis. In 2019, National Institute for Clinical Excellence guidelines (UK) suggested offering radioactive iodine as first-line definitive treatment for adults with Graves' disease, toxic nodular and multinodular goitre unless it is unsuitable or anti-thyroid drugs are likely to achieve remission. The aim of this study was to evaluate the clinical outcome 1 year after using fixed-dose Iodine-131 for hyperthyroidism and time (months) to response.

METHODS

Electronic patient records were studied retrospectively for all patients who were treated with radioiodine therapy for hyperthyroidism between July 2017 and March 2020 at a district general hospital in the UK. Response to radioiodine therapy was initially assessed at 6 weeks and then at 3, 6, and 12 months by a dedicated thyroid nurse-led virtual follow-up.

RESULTS

We identified 67 patients with a mean age of 55.9 years (range 18-84); 48 (72%) females, 19 (28%) males. Of these, 57 (85%) patients were cured at 12 months (93.7% in non-Graves', 82.3% in Graves' group). Gender, diagnosis and pre-treatment disease interval were not significantly associated with treatment success. Non-Graves' patients had a significantly shorter time to discharge than Graves' patients receiving Iodine-131 (8 versus 10.3 months, p = 0.0174).

CONCLUSION

The results of the present study demonstrate that a single fixed dose of Iodine-131 therapy is highly effective and comparable to outcomes from calculated dose therapy in literature. We propose the routine use of scoring system to calculate risk of relapse for all newly diagnosed hyperthyroid patients to tailor treatment modality.

Radioiodine therapy of Graves' disease

Graves' disease (GD), the most common cause of hyperthyroidism, is an autoimmune disease directly caused by circulating autoantibodies that bind and activate the TSH receptor, inducing metabolic activation of the thyroid gland; this may be associated with important cardiac (atrial fibrillation) and ocular (ophthalmopathy) complications. Treating GD with real curative intent implies the full elimination of the functioning thyroid parenchyma using surgery or radioactive iodine therapy (RAI). RAI has been used in humans with hyperthyroidism since 1941, thanks to the pioneering work of a physician (Dr. Saul Hertz) and a physicist (Dr. Arthur Roberts). The rationale of RAI is based on the effect of radiation of ¹³¹I on target cells leading to DNA damage, both directly, through breakage of molecular bonds, and indirectly through the formation of free radicals. In particular, irradiation causes a broad spectrum of cellular damage due to the production of reactive oxygen species and lipid peroxidation of the plasma membrane. Thus, RAI-related cellular death takes place through both apoptosis and necrosis. The aim of this review was to summarize indications, efficacy, safety profile, and dosimetric aspects of RAI treatment in patients affected by GD.

Use of Adjunctive Therapy to Achieve Preoperative Euthyroidism in Graves' Disease: A Case Report

BACKGROUND Graves' disease is an autoimmune disease of the thyroid gland and it is considered the most common cause of hyperthyroidism. It is characterized by particular eye manifestations, skin changes, and pretibial myxedema in addition to the signs and symptoms of hyperthyroidism. Graves' disease can be diagnosed based on clinical presentation and low thyroid stimulating hormone (TSH) and elevated free T4 (FT4) levels. Presence of TSH receptor antibody (TRAb) in the serum confirms the diagnosis of Graves' disease. Imaging studies like radioactive iodine scan will show a high and diffuse uptake. Graves' disease can be managed with three different treatment modalities: antithyroid medications, radioactive iodine, or surgical removal of the thyroid gland. Whenever surgery is indicated, careful preoperative management to achieve euthyroidism is needed to optimize the surgical outcome. CASE REPORT This is a case of a 37-year-old Saudi male known to have Graves' disease for 2 years who presented to the endocrine surgery clinic with neck swelling, difficulty breathing, and change in voice. After multiple attempts to control his fluctuating thyroid levels, the team eventually managed to achieve a euthyroid state in the patient with the addition of saturated solution of potassium iodide (SSKI), and thus rendering him eligible for urgent surgery. CONCLUSIONS We report this case to show that SSKI can be used as adjunctive therapy to achieve a preoperative euthyroid state in refractory Graves' disease.

Effect of Different 131I Dose Strategies for Treatment of Hyperthyroidism on Graves' Ophthalmopathy

PURPOSE

The study aims to define the effect of different dose strategies on ophthalmic complications in patients with Graves' disease (GD).

METHODS

All the patients with GD and no or inactive ophthalmopathy (clinical activity score; CAS < 3) underwent Snellen chart examination, measurement of proptosis, thyroid volume, and radioactive iodine uptake, and randomized into 1 of 3 groups. In group 1, all the patients received fixed low dose (FLD) of 259 MBq of I, whereas in group 2, all the patients received fixed high dose (FHD) of 555 MBq, and in group 3, calculated dose (CD) was administered to deliver 5.55 MBq/g (thyroid weight) of I. All examinations were repeated 6 months after treatment. The measurement of thyroid function tests and clinical examination were repeated after 12 months.

RESULTS

We studied 92 patients (58 female and 34 male) with mean age of 38.2 ± 12.0 years. Overall, 29, 32, and 31 patients were studied in FLD, FHD, and CD groups, respectively. The patients in CD received a mean activity of 240.5 MBq. The 3 groups were not significantly different regarding age, sex ratio, radioactive iodine uptake, smoking, visual acuity, and proptosis. The response rate 12 months after radioactive iodine therapy was 66.7%, 94.4%, and 92.9% in FLD, FHD, and CD groups, respectively (P = 0.05). Overall, CAS was increased significantly after treatment. Delta proptosis and delta CAS were increased significantly in FHD group compared with other groups (P < 0.05). The highest increment in proptosis was seen in FHD group.

CONCLUSIONS

The administration of 5.55 MBq/g of I has fewer ophthalmic complications compared with high fixed dose model and is more effective than low fixed dose strategy.

The success rate of radioactive iodine therapy for Graves' disease in iodine-replete area and affecting factors: a single-center study

OBJECTIVE

Initial treatment options for Graves' disease include antithyroid drugs, radioactive iodine (RAI), and surgery. Because of similar effects of three modalities, initial treatment preferences differ by country to country. In Korea, iodine-replete area, RAI was used as a second-line option. For these reasons, the RAI success rate in Korea might differ from other countries.

METHODS

A total of 247 patients were enrolled. The primary outcome was cumulative success rate in the first year, and the secondary outcome was factors that affected the success. Delayed response, which included patients who attained successful RAI during the follow-up, after the first year without further RAI, and factors that affected the delayed response were reviewed.

RESULTS

The cumulative success rate in the first year was 62.8%. Higher RAI activity [odds ratio (OR) 2.56, P = 0.02], longer disease duration (OR 0.43, P = 0.01), larger goiter (OR 0.31, P = 0.01), and higher post-RAI fT4 (fT4 between upper normal limit (UNL) to 1.5 × UNL; OR 0.24; P < 0.01, 1.5 × UNL<; OR 0.08; P < 0.01) were associated with RAI success. Twenty-seven patients showed delayed response, and goiter size (P < 0.05), pre-RAI TSH receptor Ab (TRAb) level (P < 0.01) and post-RAI fT4 (P < 0.01) were associated.

CONCLUSION

The success rate of the first RAI in Korea was lower than that in other countries. Delayed response was observed in patients with smaller goiter, low pre-RAI TRAb, and low post-RAI fT4. In these patients, clinical follow-up with monitoring could be an option, and decision of optimal timing of first RAI is crucial.