Color flow doppler sonography (CFDS) is a powerful technique which displays tissue blood flow and vascularity. Hyperthyroidism due to Graves' disease is characterized by variable degrees of increased blood flow at CFDS. The purpose of this study was to evaluate CFDS patterns in five women with thyrotoxicosis factitia, a condition due to surreptitious ingestion of excess thyroid hormone. Diagnosis was supported by the finding of elevated free thyroxine (FT4), ranging 24.2-67.6 pmol/L (normal values: 8.3-20.5), elevated free triiodothyronine (FT3), ranging 9.9-26.7 pmol/L (normal values: 3.8-8.4), undetectable thyrotropin (TSH), absent anti-thyroid antibodies, undetectable serum thyroglobulin (Tg) concentrations, very low/suppressed thyroidal radioiodine uptake and normal/low urinary iodine excretion. Moreover, all patients admitted thyroid hormone pills intake. All patients had normal thyroid volume and echogenicity at conventional sonography (mean estimated volume, 9.4 ml, range, 6-11 ml), and absent hypervascularity or minimal intrathyroidal vascular spots at CFDS. The peak systolic velocity (PSV) was at the lower limit of normal values (mean, 4 cm/sec, range 3-5 cm/sec). Twenty-six women with untreated Graves' disease had an increase in the mean PSV, (mean 12.9 cm/sec, range 8-20, p < 0.001) and diffuse hypervascularity. CFDS pattern in 24 normal women residing in the same area did not differ from that found in patients with thyrotoxicosis factitia. Thus, due to the nonthyroidal origin of excess thyroid hormone, CFDS showed absent hypervascularity and normal PSV in spite of a thyrotoxic status. These findings well correlate with the etiology of thyrotoxicosis factitia and may represent an additional, useful tool to confirm the diagnosis. For its easiness, rapidity (10 min) and noninvasive features, CFDS can be considered a first line test during office examination when thyrotoxicosis factitia is suspected.