Thallium imaging is increasingly being used to evaluate the thyroid. Uptake patterns of 201Tl in benign and malignant thyroid nodules have been described. Thallium localizes all thyroid tissue with possibly different rates of washout in benign and malignant nodules. This case demonstrates a follicular adenoma presenting 123I as a clinically palpable nodule with nonvisualization of the remainder of the thyroid gland consistent with the diagnosis of an autonomously hyperfunctioning module. Subsequent thallium scanning revealed a complete reversal of tracer distribution with lack of uptake of 201Tl in the nodule while the rest of the gland showed normal thallium accumulation. Surgical excision of the nodule demonstrated follicular adenoma.