Background: I-123 or I-131 labeled metaiodobenzylguanidine (MIBG) scintigraphy is an important imaging modality for evaluating childhood neural crest tumors such as neuroblastoma. Protection of the thyroid from radiation from free radioactive iodine by premedication with stable iodine is essential but thyroid uptake has still been reported in a considerable proportion of patients undergoing diagnostic I-123 MIBG and post-therapeutic I-131 MIBG scintigraphy. Reports in diagnostic I-131 MIBG scintigraphy are few.

Objective: To determine prevalence of thyroid dysfunction and factors associated with thyroid uptake in patients who underwent diagnostic I-131 MIBG scintigraphy.

Materials and Methods: Diagnostic I-131 MIBG scintigrams were reviewed for presence or absence of thyroid uptake. Factors potentially affecting thyroid uptake including age, weight, height, and history of prior thyroid uptake were investigated.

Results: Thyroid uptake was found in 23.6% of 161 I-131 MIBG scintigrams from 55 patients and was associated with older age (uptake 8.5±7.0 years, no uptake 5.4±3.3 years, p=0.007), and greater body weight (uptake 23.5±12.6 kg, no uptake 18.4±9.0 kg, p=0.014). Prior thyroid uptake increased odds of having thyroid uptake (OR 5.37, 95% CI 2.11 to 13.69, p<0.001). Five of nine patients who received thyroid function testing had subclinical hypothyroidism, which did not seem to relate to thyroid uptake. Overt hypothyroidism was not found.

Conclusion: Thyroid uptake was found in 23.6% of diagnostic I-131 MIBG scintigrams. Scintigrams with thyroid uptake are of patients who were older and had greater body weight. Odds of having thyroid uptake were increased if prior thyroid uptake was present. Subclinical hypothyroidism was found but did not seem to relate to thyroid uptake.

Keywords: Diagnostic I-131 MIBG scintigraphy, Hypothyroidism, Metaiodobenzylguanidine scintigraphy, MIBG scintigraphy, Thyroid diseases