Introduction: Radioactive iodine ((131)I) is widely prescribed for treatment of Graves' disease. A dose of 370 to 555 MBq (10 to 15 mCi) is usually enough, but reports of improved remission rates with single doses up to 20-30 mCi, and 38.5 mCi at most, exist.
Case presentation: A 53-year-old male patient was evaluated in September 2005, with symptoms of thyrotoxicosis for 2 years. He presented with tachycardia (130 bpm) and a large goiter. Thyrotropin was <0.01 uIU/ml (0,41-4,5), free thyroxin >7.77 ng/dl (0.9-1.8), anti-thyreoperoxidase antibody: 374 IU/ml (<35) and anti-thyroglobulin antibody: 749 IU/ml (<115). Ultrasound: diffuse goiter, no nodules; right lobe: 7.9 x 3.8 x 3.8 cm; left: 7.7 x 3.5 x 3.8 cm; isthmus: 1.6 cm. Propylthiouracil 300 mg t.i.d. and propranolol were prescribed. Thyroid (99m)Tc-pertechnetate uptake: 52% (0.35-1.7%) and estimated thyroid volume: 149 mL. After 30 days, he received 555 MBq (15 mCi) of (131)I-iodide. Six months after radioiodine therapy, under methimazole 40 mg, thyroid stimulating hormone was 1.5 uIU/ml; free thyroxine 0.54 ng/dl. Methimazole was suspended. In 21 days, thyroid stimulating hormone was 0.03 uIU/ml; free thyroxine 0.96 ng/dl. Methimazole was reintroduced. One year later, thyroid stimulating hormone was <0.01 uIU/ml and free thyroxine >7.77 ng/dl. Thyroid (99m)Tc-pertechnetate uptake was 45% and estimated thyroid volume 144 mL. A 1110 MBq (30 mCi) radioiodine therpy was administered. He used Methimazole for 8 months, when overt hypothyroidism appeared (TSH: 25.30 uIU/ml; free thyroxine: 0.64 ng/dl). Methimazole was interrupted. Hyperthyroidism returned 6 weeks later (thyroid stimulating hormone <0.01 uIU/ml; free thyroxine >7.77 ng/dl). Thyroid (99m)Tc-pertechnetate uptake was 25% and estimated thyroid volume 111 mL. Methimazole was prescribed again. In March 2008 he received a 2590 MBq (70 mCi) radioiodine therapy. By may/2008, under methimazole 20 mg, his TSH was 0.07 uIU/ml; free thyroxine 1.31 ng/dl. In October 2008 he presented overt hypothyroidism (TSH 91.6 uIU/ml; free thyroxine 0.34) and was given levothyroxine 75 mcg/day. He remains euthyroid under hormone replacement.
Conclusion: Our presented case of Graves' disease received a cumulative dose of 4255 MBq (115 mCi). The high uptake could indicate accelerated iodine turnover with (131)I short time of action. Impaired hormone synthesis could also be present. We believe the extremely high dose required was due to the initial very high iodine uptake and large thyroid volume.