The Clinical Significance of an Isofunctioning Thyroid Nodule

The management of an “isofunctioning” thyroid nodule—defined by radioiodine uptake equivalent to normal thyroid tissue—is often a dilemma. Our goal was to determine the percentage of thyroid nodules that were isofunctioning and the frequency with which carcinoma occurred in an isofunctioning nodule. Patients referred for evaluation of a thyroid nodule from 1990 to 2002 were reviewed and those with an iodine-123 thyroid scintiscan were identified. Nodule size, serum thyrotropin, fine-needle aspiration biopsy, pathology, and follow-up were determined for patients with an isofunctioning, hypofunctioning, or hyperfunctioning thyroid nodule. Of the 562 patients with a thyroid nodule 273 (49%) had a thyroid scan. Nodules were hypofunctioning in 232 (85%), isofunctioning in 29 (11%), and hyperfunctioning in 12 (4%) patients. Mean nodule size and serum thyrotropin level were respectively 2.49 ± 0.23 cm and 1.73 ± 0.26 μIU/mL for isofunctioning, 2.47 ± 0.31 cm and 0.83 ± 0.21 μIU/mL for hyperfunctioning, and 3.94 ± 0.13 cm and 1.86 ± 0.28 μIU/mL for hypofunctioning nodules. Seventeen patients with an isofunctioning nodule underwent thyroidectomy and 12 patients were followed for a mean 27 months without an increase in nodule size. No patient with a hyperfunctioning nodule, six patients (21%) with an isofunctioning nodule, and 44 patients (19%) with a hypofunctioning nodule had a carcinoma. We conclude that the risk of malignancy in an isofunctioning nodule is similar to that of a hypofunctioning nodule and therefore the management should be based on routine fine-needle aspiration biopsy.