To determine whether percutaneous needle aspiration of the thyroid affects tests of thyroid function, in particular thyroglobulin (TG), serum TG, T4, free T4 index, T3, and TSH were measured before and after percutaneous needle aspiration biopsy of the thyroid in 25 subjects. Seven control subjects were tested before and after vigorous external manual palpation of the thyroid. Serial measurements were made additionally in 3 subjects undergoing thyroid surgery to assess how quickly serum TG increases after injury. The results were analyzed, and statistically significant differences between paired results were defined if the differences were greater than the maximum interassay variation in 11 consecutive assays. Eleven out of 25 patients had statistically significant elevations of serum TG after aspiration. None of the seven who underwent external manual palpation of the thyroid gland had elevation of serum TG. With three exceptions, there were no significant changes in serum T4, free T4 index, T3, and TSH in either group. Marked elevations in serum TG occurred within 2 min after open manual palpation, diathermy, and excision. The results were analyzed and correlated with factors that might lead to release of TG from the thyroid during needle aspiration. No positive correlation was observed with the apparent degree of trauma, the size of nodule, the TG content, volume or character of aspirate, or the time elapsed from aspiration to withdrawal of the blood sample. Correlation of serum TG elevation with final diagnosis did not show a significant trend; however, the existence of a possible relationship needs further studies. We conclude that serum for TG measurement should be obtained before percutaneous thyroid aspiration biopsy.