A Case for Lithium Pretreatment Prior to Radioactive Iodine Ablation in Grave’s Disease

Abstract

Radioactive iodine ablation (RAIA) therapy with Iodine-131 (I-131) is an established treatment for grave’s thyrotoxicosis. However, there is 10 to 20% chance of treatment failure. Lithium, a drug used to treat bipolar disorder, has significant effects on thyroid function. The most clinically relevant is the inhibition of thyroid hormone release. It is also known to inhibit colloid formation, and is involved in blocking organic iodine as well as thyroid hormone release from the thyroid gland without an effect on radioiodine uptake. This leads to increased radioiodine retention in the thyroid gland. Here, we present a case which exemplifies this action of lithium. A 46 year old male with a history of atrial fibrillation and grave’s disease presented to the endocrine clinic. TSH was <0.01 and FT4 was 36. RAI uptake (RAIU) scan showed diffusely increased uptakes with 4 and 24 hour values of 61.2 and 54.6 %. He subsequently underwent RAI ablation with 18 mCi of I 131. He then presented three years later with persistent hyperthyroid symptoms. TSH <0.01 and FT4 4.3. RAIU showed 24-h thyroid uptake of 41%. Patient opted for a second treatment with RAIA and was treated with 30 millicuries of I 131. He however continued to have clinical and biochemical evidence of thyrotoxicosis and was started on methimazole (MMI). Although he was biochemically euthyroid on MMI, he continued to complain of hyperthyroid symptoms such as palpitations, tremors and weight loss. When methimazole was briefly held six months after initiation, TSH was undetectable and FT4 had increased from 0.83 to 1.42. He subsequently underwent a third RAIU off MMI which showed normal 4 and 24 hour uptake, measuring 15.7% and 28% respectively. Patient subsequently opted for third trial of RAI ablation with lithium pretreatment. He declined surgery. He was started on lithium 900mg/day for 6 days, starting on the day of RAI ablation. He underwent RAI ablation with 45 mCi I-131. Patient tolerated the procedure well with subsequent tests indicating hypothyroidism requiring levothyroxine supplementation. Patient’s hyperthyroid symptoms resolved. Several factors affect the efficacy of radioiodide therapy for hyperthyroidism including the short persistence of radioiodide in the thyroid gland. In hyperthyroid Graves’ patients, radioactive iodide uptake is enhanced due to presence of TSH receptor antibody, however, radioiodide is also rapidly discharged because of its increased turnover. Lithium can significantly reduce the release of iodine from the thyroid gland and thus increase iodine retention. There is evidence to suggest that adjuvant lithium can increase thyroidal radioiodine uptake in patients with a low baseline RAIU (< 30%). This case demonstrate that lithium can be used safely prior to RAI therapy in cases of RAI ablation failure even with low baseline RAIU.