The thyroid scan shown below exhibits a pattern that is most consistent with which of the following disorders?

The thyroid scan illustrated in the question shows a single focus of increased isotope uptake, often referred to as a "hot" nodule; the remainder of the thyroid gland has not taken up radioactive iodine. Hyperfunctioning adenomas become independent of thyroid stimulating hormone (TSH) control and secrete thyroid hormone autonomously, which results in clinical hypehyroidism. The elevated thyroid hormone levels ultimately diminish TSH levels severely and thus depress function of the remaining normal thyroid gland. An isolated focus of increased uptake on a thyroid scan is viually diagnostic of a hyperfunctioning adenoma. Carcinomas usually display diminished uptake and are called "cold" nodules. Graves' disease would probably manifest as a diffusely hyperactive gland without nodularity. Multinodular goiter would display many nodules with varying activity.