title: Association between excessive urinary iodine excretion and failure of radioactive iodine thyroid ablation in patients with papillary thyroid cancer.
abstract: BACKGROUND
  A low-iodine diet (LID) is usually recommended for a week or two before radioactive iodine (RAI) ablation therapy in papillary thyroid cancer (PTC) patients after total thyroidectomy. However, it is still controversial whether an LID affects ablation outcomes. We therefore evaluated the association between urinary iodine excretion and the rate of successful ablation and investigated the determinants of successful RAI ablation outcomes.
  
  
  METHODS
  We retrospectively reviewed the records of 295 consecutive patients with PTC who received 1110 MBq RAI remnant ablation therapy with thyroid hormone withdrawal after total thyroidectomy. Successful ablation was defined as either no visible or faint uptake on a follow-up scan (definition 1), or no visible or faint uptake on a follow-up scan and a stimulated thyroglobulin level <2 ng/mL (definition 2).
  
  
  RESULTS
  The proportion of patients with appropriate LID status (defined as a urinary iodine concentration [UIC] <66.2 μg iodine/g creatinine [μg/gCr]) was significantly higher in the successfully ablated group (81% vs. 67%, p=0.03). Based on definition 1, 80.3% (237/295) of patients were successfully ablated. The ablation rate was significantly lower in patients who had a UIC >250 μg/gCr at the time of RAI ablation (p<0.05). In multivariate analysis, a UIC >250 μg/gCr was the only significant variable associated with ablation failure (p=0.002, odds ratio [OR] 4.74 [95% confidence interval (CI) 1.78-12.63]). Based on definition 2, 74.9% (221/295) of patients were successfully ablated. A UIC >250 μg/gCr at RAI administration showed a significant association with ablation failure (p<0.05). The OR of a UIC >250 μg/gCr for ablation failure was 3.88 [CI 1.42-10.57] (p=0.008).
  
  
  CONCLUSIONS
  Excessive iodine intake (UIC >250 μg/gCr) was associated with poor RAI ablation outcomes. Because this amount of iodine is very high, we propose that the level of strictness of the LID protocol should be modified according to the region that the patient is from and the food that the patient is accustomed to eating. Even in those areas where iodine intake is high, overly strict compliance with an LID protocol is not necessary and simple recommendations to avoid iodine-rich foods would be appropriate.
authors.affiliation: Division of Endocrinology and Metabolism, Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Republic of Korea.
authors.forename: Seo Young
authors.initials: SY
authors.lastname: Sohn
authors.forename: Joon Young
authors.initials: JY
authors.lastname: Choi
authors.forename: Hye Won
authors.initials: HW
authors.lastname: Jang
authors.forename: Hye Jeong
authors.initials: HJ
authors.lastname: Kim
authors.forename: Sang Man
authors.initials: SM
authors.lastname: Jin
authors.forename: Se Won
authors.initials: SW
authors.lastname: Kim
authors.forename: Sunghwan
authors.initials: S
authors.lastname: Suh
authors.forename: Kyu Yeon
authors.initials: KY
authors.lastname: Hur
authors.forename: Jae Hyeon
authors.initials: JH
authors.lastname: Kim
authors.forename: Jae Hoon
authors.initials: JH
authors.lastname: Chung
authors.forename: Sun Wook
authors.initials: SW
authors.lastname: Kim
mesh_terms: D000328:Adult; D002277:Carcinoma / Q000473:pathology / Q000532:radiotherapy / Q000601:surgery; D002291:Carcinoma, Papillary / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery / Q000652:urine; D004032:Diet / Q000009:adverse effects*; D005260:Female; D006801:Humans; D007455:Iodine / Q000009:adverse effects* / Q000652:urine; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D008297:Male; D008875:Middle Aged; D019275:Radiopharmaceuticals / Q000627:therapeutic use*; D018714:Radiotherapy, Adjuvant; D056910:Republic of Korea; D012189:Retrospective Studies; D062606:Tertiary Care Centers; D000077273:Thyroid Cancer, Papillary; D013964:Thyroid Neoplasms / Q000473:pathology / Q000532:radiotherapy* / Q000601:surgery / Q000652:urine; D013965:Thyroidectomy; D017211:Treatment Failure; D047368:Tumor Burden; D015854:Up-Regulation
chemical_list: D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals; D007455:Iodine