Hypothyroid Hashimoto's thyroiditis with scintigraphic and echo-color Doppler features mimicking autonomous adenoma

We investigated a 48-yr-old woman on L-T4 therapy (100 microg/d) for primary autoimmune hypothyroidism, diagnosed 15 yr earlier, presenting a firm oval lump in the right thyroid lobe and symptoms of mild thyrotoxicosis. Free T4, free T3, TSH, anti-thyroperoxidase, anti-TG and anti-thyroid microsomal antibodies were determined. Thyroid US and color flow Doppler sonography (CFDS), 99mTechnetium (99mTc), radioiodine scintiscan and US guided fine needle aspiration cytology (FNAC) were performed. On L-T4 therapy, thyroid function tests showed subclinical hyperthyroidism with high anti-thyroid antibody titers. Thyroid US and CFDS revealed a voluminous hypoechoic hypervascularized nodule with increased peak systolic velocity (type III pattern) in the right lobe; the extranodular tissue volume was markedly reduced and hypoechoic. The presence of an autonomous functioning nodule associated to Hashimoto's thyroiditis (HT) was suspected, L-T4 therapy was temporary withdrawn, and the patient re-evaluated 2 months later. Off L-T4 therapy, thyroid function tests revealed marked primary hypothyroidism, while thyroid US and CFDS were unchanged. 99mTc thyroid scan showed a focal increased uptake corresponding to the nodule in the right lobe with nearly absent uptake in the remaining thyroid tissue. Only a faint, patchy thyroid distribution of 131I was detected by radioiodine scan, and RAIU was very low. Cytological examination by FNAC revealed normal follicular cells and several lymphocytes. The final diagnosis was therefore hypothyroid HT with pseudo-nodular thyroid tissue of the right lobe. To our knowledge, this is the first report of HT mimicking both scintigraphic and CFDS features of an autonomous functioning nodule.