Low-iodine diet reduces therapeutic 131-I activities prescribed for treatment of hyperthyroidism

Objectives The utility of low-iodine diet (LID) for thyroid cancer patients (pts) is well known, however its role in hyperthyroidism is not well established. We assessed the effect of 1 week LID preparation on 24h 131-I thyroidal uptake (RAIU) and prescribed therapeutic 131-I activity.

Methods 18 hyperthyroid (14 Graves’ and 4 toxic goiter) pts (16F; 2M, mean age 44 ± 19 y, range 15-83) underwent RAIU before (RAIU-1) and after (RAIU-2) LID preparation. Thyroid mass was estimated relative to the normal size (20 g) by palpation. We used a therapeutic 131-I activity calculated to deliver a dose of 0.2 mCi/g thyroid tissue retained in the gland at 24h according to the formula: I-131 activity (mCi) = 0.2 mCi/g x estimated mass RAIU The % change in RAIU, and calculated 131-I activities before and after LID were analyzed. All patients were treated with 131-I activity calculated based on RAIU-2.

Results Mean RAIU-1 was 0.36 ± 0.13, range 0.16-0.54 (normal 0.07-0.3). Mean initial calculated 131-I activity was 29 ± 14 mCi. After LID, mean RAIU-2 was 0.52 ± 0.15, range 0.33-0.83. Mean final prescribed 131-I activity was 20 ± 5 mCi. Follow-up at 6-12 months demonstrated resolution of hyperthyroidism in all (hypothyroidism in 14, and euthyroidism in 4) pts. RAIU increased in all pts (mean 58%, range: 8-188%) after LID preparation (t-test: p = 0.0007). The wide variation of %change in RAIU may reflect poor compliance with LID, or exposure to non-dietary iodine sources (e.g. skin disinfectants, hair dyes).

Conclusions Preparation with LID for 1 week in hyperthyroid pts permitted optimization of 131-I treatment by lowering the mean prescribed 131-I activity by 32% (p=0.0058). Therapeutic 131-I activity was calculated based on thyroid tissue mass and RAIU with the goal to eliminate hyperthyroidism and reduce the radiation dose to normal tissues to as low as reasonably achievable (ALARA). LID preparation allows prescription of lower 131-I activities while maintaining therapeutic effectiveness and minimizing the radiation dose to non-thyroidal tissues.