Anionic complex-carbohydrate units of human thyroglobulin. Human thyroglobulin (hTG) contains sulfate in chondroitin 6-sulfate chains and in complex carbohydrates. In this study the sulfate-containing complex carbohydrates were characterized by the number of sulfate and sialic acid residues that they contain. Samples of normal and nodular thyroid tissue were incubated for 16 h in [35S]sulfate-containing medium, and hTG was purified from the tissues and the media. Complex carbohydrates were enzymatically removed from hTG. Subsequent analysis on an HPLC anion exchange column at pH 2.2 separated the carbohydrate units according to their number of negative charges. Sulfate-containing peaks were monitored by radioactivity, and sialic acid-containing peaks were identified by their shift to lower charge after treatment with neuraminidase. Peaks corresponding to sialic acid-free carbohydrate units with one to four sulfates were identified. Also, carbohydrate units with two and three negative charges containing both sulfate and sialic acid were present. In the nodular tissue of one patient there were more sulfated units with higher charge, especially units containing sialic acid. In this patient the proportion of sulfated polyvalent units with sialic acid was 22.4% for normal and 64.6% for nodular tissue. No difference in the composition of the charged units between the tissues and their corresponding media was seen, making it unlikely that the sulfate-containing carbohydrates play a role in hTG release. It is concluded that hTG contains complex carbohydrate units with up to four sulfate groups and units with both sulfate and sialic acid. In some patients, the sulfate-containing anionic carbohydrate units of hTG from normal and nodular thyroid tissue are different. 