Efficacy of Low Doses of Radioiodine in the Treatment of Autonomous Thyroid Nodules: Importance of Dose/Area Ratio

Radioiodine (¹³¹I) represents an interesting alternative to surgery in the treatment of autonomously functioning thyroid nodules (AFTN), but leads to a significant incidence of hypothyroidism when high doses are used. Over 4 years, we have treated 40 patients (hyperthyroid [Plummer's disease]: 6, single hot nodules with undetectable thyrotropin [TSH] and normal serum free thyroxine [FT₄]: 34), 34 single hot nodules with undetectable thyrotropin TSH and normal serum free thyroxine [FT₄] with ¹³¹I. The dose level was neither related to the concentration of FT₄ nor to the iodine uptake on thyroid scintigram. Retrospectively we measured the nodule's area on the scan and calculated the dose/area ratio (DAR). Three months after treatment, 30 patients were euthyroid, 9 were still hyperthyroid, and 1 was hypothyroid. The mean DAR of the euthyroid patients was twofold higher than for the hyperthyroid subjects (1.4 ± 0.8 vs. 0.7 ± 0.3 mCi/cm²; p =.003) and one-half the DAR for the hypothyroid patient (2.82 mCi/cm²). Twenty of the 30 euthyroid patients had received a dose higher than 1 mCi/cm² and 7 of 9 hyperthyroid patients had received a dose lower than 1 mCi/cm². (X² = 12.9; p =.02). The initial values of T₄, TSH, and dose level of patients who were euthyroid or hyperthyroid at 3 months were not different. These data suggest that the efficacy of ¹³¹I for treating AFTN depends on the DAR, rather than the initial T⁴ value or the ¹³¹I uptake. A DAR between 1 and 1.5 mCi/cm² seems to be optimal and avoids hypothyroidism.