The feasibility of low-dose oral lithium therapy and its effect on thyroidal radioiodine uptake, retention, and hormonal parameters in various subcategories of hyperthyroid patients: a pilot study.

Background: Radioactive iodine (I) (RAI) is used widely for the treatment of hyperthyroidism either as a first-line treatment or following relapse after antithyroid drug treatment. Intrathyroidal retention of RAI is considered an important determinant of its effectiveness, which is believed to be prolonged by lithium.

Aims And Objectives: To study the impact of low-dose oral lithium therapy on RAI uptake and retention parameters in different subgroups of hyperthyroidism patients, and thus explore its potential role in enhancing the therapeutic efficacy of RAI in these groups of patients.

Materials And Methods: A total of 28 patients (age range=18-70 years) who were being considered for RAI therapy were included in this prospective pilot study. The patients were divided into two groups: (i) those who had not received any RAI therapy before were included in 'group I' (n=22), whereas (ii) 'group II' (n=6) included patients who were found to be persistently hyperthyroid on biochemical and clinical follow-up despite previous RAI therapy for hyperthyroidism. Patients in group I were further divided into four subgroups on the basis of the underlying etiopathology: (a) subgroup Ia - diffuse toxic goiter (n=15), (b) subgroup Ib - autonomous functioning module (n=2), (c) subgroup Ic - toxic multinodular goiter (n=4), and (d) subgroup Id - nontoxic multinodular goiter (n=1) on the basis of scintigraphic and clinical findings. All patients first underwent 25 μCi I uptake estimation at 2, 24, and 48 h and values thus obtained were considered the baseline for further evaluation. After biochemical assessment of normal renal and liver functions, patients received 900 mg lithium per day in three divided doses orally, and on the fourth day after starting tab lithium, the serum lithium level was estimated with continued lithium administration. On the fifth, sixth, and seventh day, patients underwent lithium-primed 25 μCi I uptake estimation at 2, 24, and 48 h. Retention index (RI) was calculated using the formula [RI=(48 h uptake-24 h uptake)/24 h uptake×100]. A day after completion of uptake study, that is, on the third day from diagnostic (25 μCi I) RAI administration, patients received a fixed 5 mCi therapeutic RAI dose after their suitability for the same was ascertained using clinical, biochemical, and scintigraphic findings as the criteria. Lithium administration was stopped 5 days after therapy.

Results: Lithium priming resulted in a significantly reduced serum FT4 level in subgroup Ia (diffuse goiter) of group I. Similarly, lithium priming resulted in a statistically significant increase in the radioiodine RI in subgroup Ia. Lithium priming resulted in increased retention of radioiodine and reduced serum FT4 level in the rest of the study population also, but the difference was not statistically significant (likely because of fewer patients in these subgroups). The low-dose lithium priming regimen used in the present study was found to be feasible and safe. The mean serum lithium concentration was 0.6 mEq/l with the dose protocol administered and hence was considered safe. Only one patient had achieved a level of 1.5 mEq/l, without any obvious side effects, and it was clinically uneventful. One patient experienced headache necessitating dose reduction.

Conclusion: The results of this study, carried out in different groups of patients with hyperthyroidism, suggested that a short course of lithium is safe and could be beneficial for hyperthyroid patients considered for RAI therapy as it increased the RAI retention in thyroid, and thus had the potential to increase the effect of RAI therapy. Alternatively, it is proposed that lithium priming could help reduce the dose of RAI administered without compromising on therapeutic efficacy, with possible potential implications for cost reduction, radiation safety precautions, and lowered radiation dose to nontarget organs.