Outcome study of fixed dose (555 MBq) radioiodine in the treatment of hyperthyroidism: Is Tc-99m neck to thigh uptake ratio predictive of therapy outcome?

  • Sandeep Gupta, Department of Nuclear Medicine & PET, John Hunter Hospital, Australia
  • Shaun McGrath, Princeton Medical Centre; Department of Endocrinology, John Hunter Hospital, Australia
  • Guy Lewis, Department of Nuclear Medicine & PET, John Hunter Hospital, Australia
  • Suresh Viswanathan, Department of Nuclear Medicine & PET, John Hunter Hospital, Australia
  • Megan Saul, Department of Nuclear Medicine & PET, John Hunter Hospital, Australia
  • Leonard Allen, Department of Nuclear Medicine & PET, John Hunter Hospital, Australia

    • Objective: The aim of this retrospective study was to ascertain the effectiveness of a fixed dose (555 MBq) of radioiodine treatment (RIT) for hyperthyroidism and the influence of various factors, including neck to thigh uptake ratio (UR) with Tc-99m thyroid scintigraphy.

    Method: 478 (M:F = 101:377) patients with hyperthyroidism (toxic multinodular goitre, TMNG, n=252; Graves’ disease, GD, n=117; solitary toxic adenoma, STA, n=48; undifferentiated disease, UD, n=61) were treated with 555 MBq of RIT. Age, sex, severity, medication, thyroid size by palpation, family history, thyroid autoantibodies and UR were recorded. Outcomes recorded were cure (either euthyroidism or hypothyroidism) or no cure (persistent hyperthyroidism) at four months.

    Results: Overall 79.5 % patients achieved cure (35.4% hypothyroid, 44.1% euthyroid) at four months. Cure rates in TMNG, GD, STA and UD were 86.9%, 61.5%, 89.6% and 75.4% respectively and the incidences of hypothyroidism were 30.2%, 46.2%, 37.5% and 34.4% respectively. On logistic regression analysis, Graves’ disease, larger goitre and more severe disease were predictors of failure (p<0.005). Uptake ratio was not found to be a significant predictor. Response of Graves’ disease was variable, with only 15.4% becoming euthyroid (46.2% hypothyroid; 38.5% hyperthyroid).

    Conclusion: A high cure rate (79.5%) was achieved. Only Graves’ disease, larger goitre and more severe hyperthyroidism were associated with poorer outcome; higher dose RIT may have nullified the effects of other variables, including uptake ratio. The variable response in Graves’ disease indicates that euthyroidism is difficult to achieve and hypothyroidism is a more likely outcome.