OBJECTIVE: Thyroid status in the months following radioiodine treatment for Graves' disease can be unstable.Our objective was to quantify frequency of abnormal thyroid function post-radioiodine and compare effectiveness of common management strategies. DESIGN: Retrospective, multi-centre, observational study. PATIENTS: Adult patients with Graves' disease treated with radioiodine with 12 months' follow-up. MEASUREMENTS: Euthyroidism was defined as both serum thyrotropin (TSH) and free thyroxine (FT4) within their reference ranges or, when only one was available, it was within its reference range; hypothyroidism as TSH ≥ 10 mu/L, or subnormal FT4 regardless of TSH; hyperthyroidism as TSH below and FT4 above their reference ranges; dysthyroidism as the sum of hypo- and hyperthyroidism; subclinical hypothyroidism as normal FT4 and TSH between the upper limit of normal and <10 mu/L; subclinical hyperthyroidism as low TSH and normal FT4 RESULTS: Of 812 patients studied post-radioiodine, hypothyroidism occurred in 80.7% and hyperthyroidism in 48.6% of patients. Three principal post-radioiodine management strategies were employed: (a) anti-thyroid drugs alone, (b) levothyroxine alone and (c) combination of the two. Differences among these were small. Adherence to national guidelines regarding monitoring thyroid function in the first 6 months was low (21.4-28.7%). No negative outcomes (new-onset/exacerbation of Graves' orbitopathy, weight gain, cardiovascular events), were associated with dysthyroidism. There were significant differences in demographics, clinical practice, and thyroid status post-radioiodine between centres. CONCLUSIONS: Dysthyroidism in the 12 months post-radioiodine was common. Differences between post-radioiodine strategies were small, suggesting these interventions alone are unlikely to address the high frequency of dysthyroidism. CLINICAL TRIAL REGISTRATION: Clinical. TRIALS: gov (identifier No. NCT01885533). This article is protected by copyright. All rights reserved.