Does low iodine diet improve radioiodine uptake in hyperthyroidism?

Objectives Starvation of total body iodine stimulates radioiodine uptake in the thyrocytes. However, effects of low iodine diet (LID) on radioiodine I-131 therapy (RAI) for hyperthyroidism has not been well investigated. The purpose of the study was to determine whether LID improves radioiodine uptake in the thyroid in Graves’ hyperthyroidism.

Methods 21 patients with Graves' hyperthyroidism (M4/F17. age 27-78yrs. average of thyroid weight 61g) were enrolled. Thyroid weight was determined using either CT or US. Anti-thyroid drugs were discontinued 3 days prior to RAI. After instruction by dietitians, patients followed low iodine diet program (< 400μg/day) from 7 days before to 3 days after RAI. Urine iodine excretion (UIE) was determined before LID and the day of RAI. Administered dose of I-131 was individually adjusted so that absorbed dose in the thyroid may exceed 100 Gy. Along with measurement of serum levels of FT3, FT4, TSH and TRAb, 24hr uptake of radioiodine normalized by thyroid weight (RIU; %/g) was determined in each patient. Relationship between UIE and RIU was evaluated.

Results UIE in patients before LID varied considerably (96-12100(μg/gCre). After LID, mean values of UIE were decreased to about 8% of those at routine dietary condition (41 vs. 511, p<0.005). RU ranged from 0.64-4.14 (average 1.81±0.55 %/g). There was a significant correlation between UIE after LID vs. RIU (r=0.88, p<0.01). Patients with low UIE (<100 μg/gCre) showed higher RIU than those with high UIE(≥100 μg/gCre). Correlation was not significant between RIU vs. FT3, FT4 or TSH, RIU vs.TRAb, or UIE vs.TRAb.

Conclusions LID for 7 days prior to RAI significantly decreased urine iodine levels in patients with hyperthyroidism. Moreover, lower UIE achieved by LID yield higher I-131 uptake regardless of thyroid weight, thyroid function or TRAb. LID should be recommended to enhance efficacy of RAI for hyperthyroidism.