Radioiodine Cures Most Thyrotoxicosis, but Hypothyroidism Often Results

The study covered in this summary was published in medRxiv.org as a preprint and has not yet been peer reviewed.

Treatment with radioactive iodine (radioiodine) was able to cure 92% of patients with thyrotoxicosis in a single-center New Zealand study of 222 patients treated in 2015.
The most common cause of thyrotoxicosis was Graves disease (66% of patients), followed by toxic nodular goiter (26%) and solitary toxic nodule (8%).
Following radioiodine treatment, nearly two thirds of patients developed hypothyroidism, especially those with thyrotoxicosis secondary to Graves disease.

The study provides information about outcomes in patients with thyrotoxicosis who received individualized radioiodine doses and adds to findings from four previous studies with fixed radioiodine doses conducted in Australia or New Zealand.
The findings suggest that although individualized dosing is pragmatic it’s also reasonable to select treatment doses from a preselected group of standard fixed doses such as 200, 400, 550 or 600, 800, and 1000 MBq, with the decision based on thyroid size and other patient characteristics.
Although many patients with thyrotoxicosis treated with radioiodine seek normal thyroid function without need for thyroid medication, these results showed that many patients with Graves disease as well as some patients with toxic nodular goiter or solitary toxic nodule needed thyroxine after radioiodine treatment. And so patients need to be well-informed about this possible outcome and physicians need to have realistic expectations for treatment outcomes.

Retrospective, observational study of 222 patients who received treatment at a radioiodine clinic in Auckland, New Zealand, in 2015, for thyrotoxicosis secondary to Graves disease in 66% of patients, toxic nodular goiter in 26%, and solitary toxic nodule in 8%.
Patients received individualized doses of radioiodine:
- 80% of patients with Graves disease received one radioiodine dose at a median first dose of 550 MBq (range, 200-1000 MBq) and a total dose of 200-2400 MBq.
- 93% of patients with toxic nodular goiter received one radioiodine dose at a median first dose of 550 MBq (range, 400-1000 MBq) and a total dose of 400-1800 MBq.
- All patients with solitary toxic nodule received one radioiodine dose (median 550 MBq; range, 500-550 MBq).
Cure of thyrotoxicosis was defined as either development of hypothyroidism with need for long-term thyroxin or persistently normal or elevated levels of thyroid stimulating hormone (TSH) in the absence of antithyroid medication for at least 1 year after radioiodine treatment.

After radioiodine therapy:
- 92% of patients with Graves disease were cured and 83% required thyroxine. Thirteen of these patients (9%) met the definition of cured after treatment, did not need thyroxine treatment, and had a normal final TSH level.
- 93% of patients with toxic nodular goiter were cured and 22% required thyroxine. Thirty-six of these patients (62%) met the definition of cured after treatment, did not need thyroxine treatment, and had a normal final TSH level.
- All patients with solitary toxic nodule were cured and 35% required thyroxine. Five of these patients (29%) met the definition of cured after treatment, did not need thyroxine treatment, and had a normal final TSH level.
At a mean of about 3 years after radioiodine treatment:
- The most recent TSH level was elevated in 31% of patients overall — in 35% of patients with Graves’ disease, 17% of patients with toxic nodular goiter, and 47% of patients with solitary toxic nodule — and in 37% of patients taking thyroxine.
- Among patients with elevated TSH, indicating suboptimally controlled hypothyroidism, 30% had a TSH level > 10 mU/L and 70% had a level of 4-10 mU/L.