Usefulness of low iodine diet in managing patients with differentiated thyroid cancer - initial results

BACKGROUND Low iodine diet (LID) is recommended in patients with differentiated thyroid cancer before radioiodine administration. Patients with increased thyroglobulin (Tg) level, but negative (131)I whole body scan present diagnostic and therapeutic dilemma. This study was designed to evaluate the benefit of a two-week LID in patients with elevated serum Tg levels and negative (131)I whole body scans. PATIENTS AND METHODS.: For the impact assessment of two-week LID on radioiodine tissue avidity, radioiodine scans before and after LID were compared. Sixteen patients with serum Tg > 2 μg/L, negative Tg-antibodies, and negative radioiodine scans underwent two-week LID before the (131)I administration. Fourteen patients underwent diagnostic scanning and two patients received radioiodine therapy. Iodine concentration in the morning urine specimens were measured in each patient, a day before and 15(th) day after starting LID. RESULTS Following self-managed LID, patients were able to significantly reduce their iodine body content by 50% (range 28-65%, p<0,001). 13 patients (82%) accomplished mild iodine deficiency (50-99 μg/L) and one patient (6%) achieved targeted moderate iodine deficient state (<50 μg/L). All diagnostic post-LID scans were negative. Both post-therapy (131)I scans showed radioiodine accumulation outside of normal (131)I distribution (neck region and diffuse hepatic uptake). This study demonstrated that two-week LID is effective way to decrease total body iodine content, although without a visible effect on post-LID diagnostic (131)I scans. CONCLUSIONS A more stringent dietary protocol and longer iodine restriction period are probably needed to achieve targeted moderate iodine deficiency in patients preparing for (131)I administration. This might result in higher radioiodine avidity of thyroid remnant/metastases.