Positive anti-thyroid peroxidase (TPO) antibodies and higher TRAb levels, albeit within the normal range, at the time of treatment are risk factors, because the incidence of hyperthyroidism was 22% (6/27) in patients with positive anti-TPO antibodies vs 2% (2/103) in those negative for anti-TPO antibodies, and the TRAb levels were higher in hyperthyroid patients than in controls at the time of treatment (3–7 vs 1–3 U/l). Here, TPO is linked to hyperthyroidism.