Several factors may predict normal thyroid function after radioiodine (RAI) therapy for hyperthyroidism, including older age, larger volume of thyroid gland, and lower iodine uptake level prior to RAI therapy, according to study results published in BMC Endocrine Disorders.
Of the possible treatment option for hyperthyroidism, RAI therapy is the most popular modality in the United States, whereas antithyroid drugs are preferred in many other areas of the world. RAI therapy is considered successful when the patient is rendered hypothyroid or euthyroid. The goal of the current study was to explore potential predictors of normal thyroid function in patients who underwent RAI treatment.
The study cohort included 144 patients (98 women, 46 men) with hyperthyroidism secondary to Graves disease (63 patients) or toxic multinodular goiter (81 patients) who were referred for RAI therapy. The study also included patients who had undergone a second treatment with RAI. Fixed doses of 131I– varied between 610 and 790 MBq.
The researchers collected laboratory, clinical, and radiologic data, including thyroid ultrasound before RAI therapy and 6 months after the treatment, as well as scintigraphy results.
All patients were observed for 12 months and at that point were divided into 3 groups according to their thyroid function test results: hypothyroidism (81 patients), normal thyroid function without medication (52 patients), or persistent hyperthyroidism (11 patients).
Multivariate logistic regression analyses revealed several factors that predicted achieving normal thyroid function tests in patients treated with RAI: age (odds ratio [OR], 1.06; 95% CI, 1.025-1.096; P =.001), thyroid volume (OR, 1.04; 95% CI, 1.02-1.06; P =.0001), and iodine uptake (OR, 0.95; 95% CI, 0.91-0.98; P =.004).
There was a significant difference in age of patients who achieved hypothyroidism (53.5±16.3 years) or had persistent hyperthyroidism (50.9±16.4 years) after RAI therapy compared with patients with normal thyroid function after RAI (67.3±12.7 years).
There was a significant association between thyroid gland volume before RAI therapy and the outcome of therapy. Hypothyroidism was more common in those with smaller baseline thyroid volume (mean volume for hypothyroid group, 22.6±13 cm3), whereas the risk for persistent hyperthyroidism was higher in those with larger thyroid glands (mean volume for hyperthyroid group, 54±31.7 cm3).
Patients who had normal thyroid function tests after RAI therapy were found to have lower iodine uptake level (30.2%±11.3%) at baseline, whereas higher iodine uptake levels were evident in patients who subsequently developed hypothyroidism (36.4%±13.4%) or had persistent hyperthyroidism (54.3%±16.6%).
When the researchers analyzed the data separately for patients with Graves disease and toxic multinodular goiter, only age (OR, 1.06; 95% CI, 1.001-1.13; P =.047) was found to be a significant predictor of achieving normal thyroid function with RAI therapy for Graves disease. In contrast, in patients with toxic multinodular goiter, there were several predictors, including age (OR, 1.04; 95% CI, 1.0-1.09; P =.048), thyroid gland volume (OR, 1.038; 95% CI, 1.009-1.068; P =.009), and iodine uptake level (OR, 0.95; 95% CI, 0.9-0.99; P =.02).
The researchers acknowledged several study limitations, including the relatively small sample size and lack of a control group. They also noted that there is a possibility of developing hypothyroidism several years after therapy in patients who are initially rendered euthyroid.
“The more advanced age, larger volume of thyroid gland and lower iodine uptake values contribute to rendering the patient euthyroid. In our opinion, physicians should take these three factors into consideration, while discussing the treatment modality with the patient, especially in case of [toxic multinodular goiter] diagnosis,” concluded the researchers.
Stachura A, Gryn T, Kałuża B, Budlewski T, Franek E. Predictors of euthyreosis in hyperthyroid patients treated with radioiodine 131I-: a retrospective study. BMC Endocr Disord. 2020;20(1):77.