Iodide ion I-123 is the most suitable isotope of iodine for the diagnostic study of thyroid diseases. Sodium Iodide I 131 Capsules Diagnostic is indicated for use in adults for: Assessment of thyroid function using radioactive iodine (RAI) uptake test and Imaging the thyroid (scintigraphy). The following adverse reaction has been described elsewhere in the labeling: Hypersensitivity Reactions. The following adverse reactions have been identified during post-approval use from Sodium Iodide I 131 Capsules Diagnostic: Gastrointestinal disorders (vomiting, nausea, and diarrhea); General disorders and administration site conditions (local thyroid swelling); Immune system disorders (hypersensitivity reactions); Skin and subcutaneous tissue disorders (itching, rash, hives, and erythema). Certain drugs and iodine-containing foods interfere with the accumulation of radioiodide by the thyroid.