Concomitant lithium increases radioiodine uptake and absorbed doses per administered activity in graves' disease: comparison of conventional versus lithium-augmented radioiodine therapy

Background: Lithium inhibits iodine and thyroid hormone release from thyroid cells, possibly increasing radioiodine retention and anti-hyperthyroid efficacy when given adjunctively to radioiodine therapy (RAI) of Graves' disease (GD). However, the literature contains limited dosimetric data regarding the influence of concomitant lithium in this setting. Methods: We retrospectively compared dosimetric variables in patients undergoing RAI with/without adjunctive lithium (n = 52 each). We assessed two low-dose, short-duration oral lithium carbonate regimens, 450 mg/d (n = 22) or 900 mg/d (n = 30), for a mean of 4.7 +/- 1.4 d starting upon RAI administration. Patients underwent diagnostic testing to measure thyroidal radioiodine uptake (RAIU) 24 h +/- 2 h after ingesting up to 5 MBq radioiodine, receiving individualized RAI activities 24 h later. Using >= 3 RAIU daily measurements starting 24 h post-RAI, researchers were able to determine the effective radioiodine half-life and absorbed dose to the thyroid; we also calculated the absorbed dose per administered activity concentration within that organ. Rates of GD cure, defined as reaching euthyroidism or hypothyroidism post-RAI, were evaluated in patients with similar to 6 months or longer post-RAI follow-up. Results: The lithium dosage subgroups had similar dosimetric values and thus are considered together. Lithium patients and controls had similar average "diagnostic" RAIU (51.1% +/- 15.7% vs. 50.6% +/- 13.8%, p = 0.820), but the former had significantly higher RAIU post-RAI (56.3% +/- 13.5% vs. 49.1% +/- 13.5%, p = 0.002), reflecting significantly greater change in the former (+16.2% +/- 30.4% vs. -1.8% +/- 16.1%, p = 0.001). Radioiodine effective half-life was non-significantly longer in lithium patients (5.43 +/- 1.50 d vs. 5.08 +/- 1.16 d, p = 0.192). The mean RAI administered activity was 27% less in lithium patients (677 +/- 294 MBq vs. 930 +/- 433 MBq, p = 0.001), but GD cure rates were similar (83% [39/47] vs. 82% [33/40], p = 0.954), possibly due to the significantly higher thyroid dose in the lithium patients, especially in thyroid gland with a volume <= 20 mL (1.04 +/- 0.44 Gy/MBq vs. 0.76 +/- 0.30 Gy/MBq, p = 0.020). Day 3 serum lithium concentrations were low (450 mg/d: 0.26 +/- 0.12 mmol/L, 900 mg/d: 0.50 +/- 0.18 mmol/L); no lithium toxicity was noted. Conclusion: Lithium augmentation may increase the RAIU and thyroid absorbed dose, permitting potentially decreased RAI activities without sacrificing efficacy. Our observations should be confirmed in a prospective, randomized trial.