Postsurgical thyroid remnant ablation with (131)I is considered standard clinical care for most cases of papillary and follicular thyroid cancer, to eliminate normal thyroid tissue which may contain microscopic cancer. Furthermore, ablation enhances the sensitivity of subsequent (131)I scanning and serum thyroglobulin (TG) measurement for the detection of recurrent or persistent disease. Low dose (131)I (30 mCi) successfully ablates thyroid remnant in 8-61% of cases. This dose can be repeated and result in overall less radiation exposure than that associated with high dose therapy (100 mCi). Ablation achieved with one or more small doses of radioactive iodine is not associated with decreased survival or cancer recurrence. According to the literature there is no difference in the 30-year recurrence rates between groups receiving low and high dose ablation therapy for well differentiated thyroid cancer without distant metastases at the time of initial therapy. The benefits of using low dose of (131)I are minimization of whole body radiation exposure, reduction in side effects and lower cost. Higher rates of successful ablation by low dose of 131I could be achieved by increasing uptake of (131)I and/or lengthening retention of radioiodine in the remnant thyroid tissue. Recombinant human thyroid-stimulating hormone (rhTSH) has been used successfully to increase radioiodine uptake. Lithium has been used to increase radioiodine retention and has been shown to be useful in the treatment of residual or metastatic cancer. The combination of rhTSH and lithium as adjuncts to 30 mCi radioactive iodine (RAI) ablation therapy in low risk thyroid cancer patients may provide a method that reduces the cumulative dose of radioiodine needed to successfully treat thyroid cancer. The specific aim of this study is to determine whether adjunct lithium carbonate improves the success rate of postsurgical ablation of thyroid remnants using low dose (131)I (30 mCi) and rhTSH in low risk patients with differentiated thyroid carcinoma. Patients with well-differentiated papillary or follicular thyroid cancer stage I or II, according to the National Thyroid Cancer Treatment Cooperative Study (NTCTCS) classification at time of surgery, will be enrolled. Eligible patients will have had a total or near-total thyroidectomy within 6 months of enrollment. This randomized, placebocontrolled, double-blind study will permit an evaluation of the risk/benefit ratio of adding lithium as an adjuvant to the already established method of administering low-dose (131)I ablation therapy, to optimize the (131)I retention. All patients will undergo diagnostic rhTSH (123)I whole body scan at the end of the study to assess the success of thyroid ablation.