Determinants of longterm outcome of radioiodine therapy of sporadic non- toxic goitre

Objective: Radioiodine treatment is effective in reducing the size of sporadic nontoxic goitre, albeit at the expense of a high incidence of postradiation hypothyroidism. The decrease in goitre size, however, is not observed in all subjects, and little is known about recurrent goitre growth after 131I therapy. The aim of the present study was to evaluate which factors determine the longterm outcome of 131I treatment in patients with sporadic nontoxic nodular goitre, in terms of changes in both thyroid size and thyroid function. Study design: Retrospective follow-up study. Patients: Fifty patients with sporadic nontoxic nodular goitre were evaluated who had been treated in our institution with 131I (mean dose 4.4 MBq/g thyroid) in the period 1988-95. Nine patients received a second dose of 131I and one a third. Median follow-up time was 41 months (range 24-115). Measurements: Thyroid function was assesed by TSH and FT4 index, and thyroid volume by ultrasound in 46 patients, by scintiscan using the Himanka formula in three and by CT-scan in one. The response to treatment was defined as a decrease in thyroid volume of greater than 13% (i.e. the mean + 2SD of the coefficient of variation of volume measurements), and recurrent goitre as an increase in thyroid volume greater than 13% after an initial response. Results: Goitre size decreased from a median value of 82 ml (range 17-325) to 37 ml (range 6-204) two years after 131I treatment, a median reduction of 49%. The decrease in goitre size was directly related to the dose of 131I (r = 0.50, P = 0.0003) and indirectly to baseline goitre size (r = - 0.35, P = 0.006). Seven patients (14%) were nonresponders, and four (8%) experienced recurrent goitre growth after 3-5 years. These 11 patients (22%) when compared to the remaining 39 responders (78%) had larger goitres with more often a dominant nodule, and had received a lower 131I dose. The efficacy of a second dose of 131I (median reduction in goitre size 37%) was comparable to the first dose. Hypothyroidism occurred in 24 patients (48%), mostly in the first two years after treatment; 11 had overt and 13 subclinical hypothyroidism. Kaplan Meier statistics indicated a probability of 58% for developing hypothyroidism after 8 years. Hypothyroid patients had a smaller initial goitre size and a higher prevalence of TPO antibodies and a family history of thyroid disease than the patients who remained euthyroid; the 131I dose did not differ between the two groups. Conclusions: The size of sporadic nontoxic goitres is reduced on average by 50% after a single dose of 4.77 MBq 131I/g thyroid. Independent determinants of the relative decrease in thyroid volume are administered 131I dose and initial goitre size. Nonresponders (14%) and those with late recurrence of goitre growth (8%) have larger goitres and more often dominant nodules than responders. Determinants of postradioiodine hypothyroidism (cumulative risk 58% after 8 years) are the presence of TPO antibodies, a family history of thyroid disease and a relatively small goitre. The implications of these findings are that the efficacy of a given 131I dose can be enhanced when administered at an earlier stage when the goitre is still smaller, albeit at the expense of an increased risk for developing hypothyroidism.