Impact of low iodine diets on ablation success in differentiated thyroid cancer: A mixed-methods systematic review and meta-analysis

Abstract

Background: Debate remains regarding whether to recommend a low iodine diet (LID) before radioactive-iodine treatment and its duration and stringency. This mixed-methods review aimed to determine if iodine status affects treatment success, the most effective diet to reduce iodine status, and how LID impacts wellbeing.

Methods: Five electronic databases were searched until February 2021. An effectiveness synthesis (quantitative studies) and views synthesis (qualitative, survey, and experience-based evidence) were conducted individually and then integrated. Quality assessment was undertaken.

Results: Fifty-six quantitative and three qualitative studies were identified. There was greater ablation success for those with an iodine status of <50 mcg/L (or mcg/gCr) compared with ≥250 (odds ratio [OR] = 2.63, 95% confidence interval [CI], 1.18-5.86, n = 283, GRADE certainty of evidence very low). One study compared <50 mcg/L (or mcg/gCr) to 100-199 and showed similar rates of ablation success (OR = 1.59, 95% CI, 0.48-6.15, n = 113; moderate risk of bias). People following a stricter LID before ablation had similar rates of success to a less-strict diet (OR = 0.67, 95% CI, 0.26-1.73, n = 256, GRADE certainty of evidence very low). A stricter LID reduced iodine status more than a less strict (SMD = -0.40, 95% CI, -0.56 to -0.24, n = 816), and reduction was seen after 1 and 2 weeks. The main challenges were a negative impact on psychological health, over restriction, confusion, and difficulty for sub-groups.

Conclusions: Although a LID of 1-2 weeks reduces iodine status, it remains unclear whether iodine status affects treatment success as only a few low-quality studies have examined this. LIDs are challenging for patients. Higher-quality studies are needed to confirm whether a LID is necessary.

Keywords: differentiated thyroid cancer; iodine status; low iodine diets; meta-analysis; mixed-methods systematic review.

Figure 1

(A) Forest plot of odds ratio and 95% CI of pooled studies comparing an iodine status <50 mcg/L (or mcg/gCr) to ≥250 on ablation success. (B) Forest plot of odds ratio and 95% CI of pooled studies comparing a stricter LID to a less‐strict LID on ablation success. CI, confidence interval; LID, low iodine diet

Figure 2

(A) Forest plot of standardized mean differences (SMD) and 95% confidence interval (CI) of pooled studies comparing the effect of low iodine diet (LID) compared with normal diet on iodine status (B) Forest plot of SMD and 95% CI of pooled studies comparing the effect of stricter LID compared to less‐strict LID on iodine status (C) Forest plot of SMD and 95% CI of pooled studies comparing the effect of LID duration on iodine status.