To evaluate the diagnostic and treatment consequences of using a routine needle biopsy procedure on all thyroid nodules without a radioisotopic scintigraphic study, 12 patients with documented hyperfunctioning thyroid nodules were retrospectively evaluated regarding the physical and cytologic observations obtained after a fine-needle (22 to 27-gauge) aspiration biopsy (FNB) procedure. Among the seven solid lesions, features of marked cellularity and nuclear pleomorphism were present in three and moderate sheets of epithelium in four raising the suspicion of underlying malignancy, while five mixed (cystic and solid) lesions were larger than 3 cm, hemorrhagic, and recurrent, with two having detectable sheets of epithelium. Evidence for concomitant lymphocytic thyroiditis was seen in five of 12 (42%) patients, and nine had positive serum antithyroid antibody titers as well. In conclusion, total reliance on FNB without scintigraphy could lead to operations on hyperfunctioning thyroid adenomas for suspected malignancy, of whom the vast majority would be benign, and could expose some unprepared patients with thyrotoxicosis to surgical morbidity. Routine thyroid hormone suppression therapy for apparently benign inflammatory or cystic degenerative lesions could also induce hyperthyroidism in patients with hyperfunctional or autonomous (nonsuppressible) nodules. When using an initial FNB approach, the need for thyroid function studies and scintigraphy before undertaking surgery or thyroid hormone feeding, as well as the consequences of omitting such studies, should be considered.