Endocrine Abstracts

The administration of radioactive iodine (¹³¹I) is widely used in the treatment of patients with hyperthyroidism. We have previously reported better cure rates in patients receiving a single dose of 600 MBq ¹³¹I compared with those treated with lower doses. We set out to evaluate if baseline patient characteristics predict which patients require multiple doses of radioiodine to induce cure. We compared 42 subjects requiring ≥3 doses with 290 patients cured with two doses and with 868 subjects cured following a single dose. The size of the first dose administered was lower (P<0.001) in those receiving multiple doses with an initial dose of 185 MBq administered to 64.2% of patients treated with ≥3 doses versus 41.2% (two doses) and 24.2% (one dose) respectively. The rates of hypothyroidism were higher in those treated with two doses (78.6%) versus subjects receiving a single dose (67.2%) but were lower in patients requiring ≥3 doses (47.6%, P<0.001)). There was a significantly higher proportion of males in those treated with ≥3 doses (31 vs 24.5% (two doses) and 19% (one dose), P=0.04). Subjects requiring a higher number of doses were significantly younger (46.9 years (≥3 doses) versus 48.3 years (two doses) versus 51 years (one dose), P=0.001) and had more severe hyperthyroidism at presentation (mean serum fT₄: 64.2 vs 51.7 vs 43 pmol/l, P<0.001). The presence of a palpable goitre was significantly higher in patients requiring 2 (84.5%) or more doses (95.3%) when compared with subjects cured following a single dose (72.6%, P<0.001). The underlying disease aetiology, smoking history, reporting of a family history of hyperthyroidism and the presence of ophthalmopathy were similar in the three groups.

Conclusions: Males, younger patients, those with higher serum fT₄ concentrations and subjects with large goitres should receive a large initial dose of ¹³¹I.