Lithium has been documented to increase retention of radioactive iodine¹³¹ I) in the thyroid of patients with Graves thyrotoxicosis [1]. By prolongation of thyroidal exposure time to ¹³¹I, lithium may increase the effectiveness of such therapy. In particular, patients with a rapid thyroid iodine turnover, a condition associated with ¹³¹I treatment failure [2], may benefit from the ¹³¹I-potentiating effects of lithium. Here, we report on the value of lithium pretreatment in a patient with apparent ¹³¹I-resistant Graves thyrotoxicosis.

A 47-year-old woman had had relapsing Graves thyrotoxicosis for 10 years. She had been treated with the antithyroid drug thiamizole in conjunction with periodic L-thyroxine substitution. Two previous ¹³¹I treatments of 10 mCi and 15 mCi had not induced remission. The patient was referred for a more definite solution. On physical examination, she had a diffuse large goiter without ophthalmopathy. After cessation of thiamizole therapy for 6 weeks, thyrotoxicosis was confirmed (thyroid-stimulating hormone level < 0.001 mIU/L; free thyroxine level, 10.5 pmol/L; free triiodothyroxine level, 26.8 pmol/L [reference values, 9.0 to 26.0 pmol/L and 3.0 to 8.4 pmol/L, respectively]). The titer of thyroid peroxidase antibodies was high. Turnover of ¹³¹I in the thyroid was high, with an uptake of 64% after 4 hours; this declined to 53% after 24 hours. Two further attempts to induce euthyroidism with 30 mCi of ¹³¹I were unsuccessful. The patient, who was unwilling to undergo thyroidectomy, agreed to an attempt to improve ¹³¹I effectiveness with lithium pretreatment. After 1 week of lithium pretreatment (1600 mg at bedtime; plasma lithium level, 0.52 mmol/L), thyroid iodine uptake, as assessed with a tracer dose of 10 µCi, corrected the high thyroidal iodine turnover.

Thyroidal ¹³¹I uptake before and after 1 week of lithium pretreatment is shown in Table 1. Subsequent treatment with a ¹³¹I dose of 30 mCi plus lithium was continued for 2 weeks and induced hypothyroidism that required L-thyroxine substitution 12 weeks after treatment. This situation has persisted for more than 2 years.

The actual radiation dose delivered to thyroid tissue is the main determinant of the effectiveness of ¹³¹I therapy [3], and a recent study showed ¹³¹I treatment failure in 50% of patients when the ¹³¹I turnover was rapid (ratio of 4- to 24-hour ¹³¹I uptake > 1.0). Despite high doses of ¹³¹I in our patient, treatment was effective only after addition of lithium. As shown by the improved 4-, 24-, and 48-hour uptake values after lithium pretreatment, the increase in retention time of ¹³¹I in the thyroid may have potentiated therapeutic effectiveness and broke through apparently ¹³¹I-resistant Graves thyrotoxicosis.

Lithium is a simple and safe treatment and has great potential as an adjunct to ¹³¹I therapy. A controlled study to define the role of lithium in patients with high thyroidal iodine turnover is indicated.