Follicular thyroid carcinoma (FTC) is the second most common type of differentiated thyroid cancer, with a high propensity for hematogenous metastasis, particularly to the lungs and bones. Radioactive iodine-131 (¹³¹I) therapy is traditionally recommended after total thyroidectomy when minimal remnant thyroid tissue is present. However, the necessity of completion thyroidectomy before ¹³¹I therapy in patients who have undergone lobectomy remains controversial. This case report describes a 73-year-old female patient with multiple distant metastases (bones, lungs, lymph nodes, and adrenal gland) that developed eight years after lobectomy for FTC. She received two courses of individualized ¹³¹I therapy (70 mCi and 200 mCi) based on pre-therapeutic assessment of iodine-avidity using ⁹⁹^mTcO4-thyroid scintigraphy. Following treatment, the patient’s pain symptoms significantly improved, and follow-up whole-body iodine scintigraphy with SPECT/CT showed marked reduction in metastatic lesions with osteoblastic repair changes. Serial thyroglobulin levels demonstrated a sustained downward trend. This case suggests that for FTC patients with iodine-avid metastatic disease after lobectomy, ¹³¹I therapy can be safe and effective, even in the presence of significant remnant thyroid tissue, provided that pre-treatment evaluation of iodine uptake capacity is thoroughly performed.