A 30-year-old female presents for evaluation of a palpable thyroid nodule. Technetium-99m (99mTc) scan demonstrates a single cold nodule. The differential diagnosis includes which of the following?

a. Carcinoma

b. A nonfunctioning adenoma

c. A thyroid cyst

d. A colloid nodule

Radioisotope scanning measures the functional activity of the thyroid gland and maps its correlation with physical findings. For routine scanning, 131I scanning has been replaced by either 123I or 99mTc. Because of the lower radiation exposure, cost and easy use, 99mTc is preferentially used. The differential diagnosis of a solitary nonfunctioning nodule includes carcinoma, colloid nodule, nonfunctioning adenoma, and cyst. The prevalence of carcinoma ranges from 5% to 20% in cold nodules and such lesions require further evaluation. If the scan detects other nonfunctioning areas in addition to the palpable nodule, the gland is at low risk for carcinoma because most patients with this finding have a multinodular goiter. A solitary, discrete area of increased activity is more often found in a young patient with an otherwise normal thyroid gland. Multiple hot spots are typically found in an older patient with a multinodular goiter.

A functioning solitary nodule that is independent of TSH is considered an autonomous nodule and can be the cause of hyperthyroidism. It is hot on scintigraphic scan. The presence of a cold nodule is insufficient information to determine a treatment plan. Although nearly all carcinomas are cold, most cold nodules are benign. Fine needle aspiration cytology is considered the most reliable means of evaluation for the diagnosis of thyroid nodules that are nonfunctional or hypofunctional by nuclide scan.