CLINICAL THYROIDOLOGY FOR PATIENTS
A publication of the American Thyroid Association

Summaries for Patients from Clinical Thyroidology by Ernest Mazzaferri, MD MACP
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THYROTOXICOSIS

What is the study about? Hospitalization for arrhythmias and cardiovascular disease may be increased in patients with hyperthyroidism treated with radioiodine.

The full article title: “Increased long-term cardiovascular morbidity among patients treated with radioactive iodine for hyperthyroidism” It is in the March 2008 issue of Clinical Endocrinology (Oxf) (Volume 68, Issue 3, pages 450-57. The authors are S Metso, A Auvinen, J Salmi, H Huhtala, and P Jaatinen. The abstract can be obtained from: http://www.ncbi.nlm.nih.gov/pubmed/17941909?dopt=Citation

What is known about the problem being studied? Some long-term studies find an increase in cardiovascular disease (CVD) mortality in patients with hyperthyroidism treated with radioactive iodine (¹³¹I). However, hyperthyroidism itself may account for the increase in CVD mortality.

What was the aim of the study? The aim of this study was to compare the rate and causes of hospitalization in patients with hyperthyroidism that were treated with ¹³¹I compared with the general population.

Who was studied? This Finnish population-based case–control study comprised 2611 patients with hyperthyroidism treated with radioiodine ⁽¹³¹I); 16.5% were men and 83.5% were women. Age and gender-matched controls were selected from a population registry.

How was the study done? The cause of hospitalization was classified into 13 general groups, which included infectious disease, gastrointestinal disease, and fracture, and CVD was further subclassified as hypertension, coronary artery disease, pulmonary circulation disorders, arrhythmias, heart failure, cerebrovascular diseases, and diseases of other arteries and cerebrovascular diseases. The rates of hospitalization were compared with that in an age-matched group in the general population.

What were the results of the study? The median age of study subjects and controls was 62 years (range, 49 to 72) and the median follow-up period was 9.0 years for the patient and control groups. For both groups, CVD was the most frequent cause of hospitalization. Still, the rate of hospitalization was higher in the patient group as compared with the control group. The risk remained elevated for as long as 35 years after ¹³¹I therapy.

Hospitalization rates for CVD were elevated only among patients treated with 7 to 10 mCi of ¹³¹I, and were about 20% higher than the general population. Patients treated with antithyroid drugs for less than 3 months or for more than 2 years were at increased risk of hospitalization due to CVD as compared with controls. Age at the time of treatment predicted an increased risk of hospitalization compared with controls, and was greatest in patients aged 60 to 98 years.

The clinical factors predicting hospitalization for CVD were toxic multinodular goiter, older age at first ¹³¹I, cumulative ¹³¹I dose, and duration of antithyroid drug therapy. Treatment with ¹³¹I was associated with a 20% increased risk of hospitalization for infectious disease, gastrointestinal disease, and for fracture, which was more common in women 50 years of age or older than in younger women or men.

How does this compare with other studies? In another study, the authors found an increased incidence of cancer in this same group after radioiodine treatment of hyperthyroidism.

What are the Limitations of this study? The main limitation of this study is the high likelihood of selection bias. Only hospitalized patients were studied leaving out those who never were hospitalized. Also it is difficult to understand how patients in this cohort treated could have an increased incidence or cancer as the result of receiving very small amounts of ¹³¹I. Moreover it is difficult to reconcile a 35 year adverse effect of a small amount of ¹³¹I. It is much more likely that the findings are due to thyrotoxicosis

What are the implications of this study? Patients with hyperthyroidism treated with radioiodine may be at increased risk of long-term morbidity and hospitalization for arrhythmias and cardiovascular disease, which it is more likely an effect of hyperthyroidism than radioiodine.

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