BACKGROUND
Graves’ disease is the most common cause of hyperthyroidism in the United States. Radioactive iodine therapy is frequently used to treat Graves’ disease. Hypothyroidism or normal thyroid function are both considered successful treatment outcomes after radioactive iodine therapy. A number of factors have been associated with rates of hypothyroidism after radioactive iodine therapy. These include the patient’s age, severity of hyperthyroidism, TSH-receptor antibody (TRAb) levels, thyroid volume and pretreatment with antithyroid drugs. While a majority of patients with Graves’ disease will have positive antithyroid peroxidase antibodies (anti-TPO) as a marker of their autoimmune disease, not all do. The authors sought to determine whether the presence of positive anti-TPO was associated with the subsequent development of hypothyroidism after radioactive iodine therapy.

THE FULL ARTICLE TITLE
Verdickt S et al 2022 TPO antibody status prior to first radioactive iodine therapy as a predictive parameter for hypothyroidism in Graves’ disease. Eur Thyroid J 11(4):e220047. PMID: 35687484.

SUMMARY OF THE STUDY
This is a study of patients with Graves’ disease who received their first radioactive iodine treatment for their hyperthyroidism between 2011 and 2019 and had anti-TPO antibodies measured. They identified 152 patients from 4 thyroid outpatient clinics in Belgium and evaluated them as to their thyroid status (hypothyroidism or cure which was either hypothyroidism or normal thyroid function) at two time points. The first was approximately 6 months after radioactive iodine therapy (using closest evaluation 2-9 months after treatment) and the second 12 months after treatment (using closest evaluation at 9-24 months).

Positive anti-TPO antibodies were identified in 105 (69%) of the patients. These patients were on average younger, had a larger thyroid gland and had more previous episodes of hyperthyroidism compared to patients with negative antibodies. In period 1, 89% of the anti-TPO– positive group and 72% in the anti-TPO–negative group developed hypothyroidism. In period 2, the observation was similar: 88% vs. 72%. This difference was statistically significant even when adjusted for previously known predictive factors such as age, degree of hyperthyroidism, TRAb titer, thyroid volume, pretreatment with antithyroid drugs and radioactive iodine dose.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
The authors concluded that positive anti-TPO antibodies are another predictor of hypothyroidism after radioactive iodine therapy for Graves’ disease. This information can help physicians and patients better understand the risks of developing hypothyroidism subsequent to radioactive iodine therapy and may be a useful took in future studies evaluating the best dose of radioactive iodine to be used.

— Marjorie Safran, MD

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