Thyroglobulin samples were prepared individually be gel chromatography from the thyroids of five persons without thyroid disease and four with goiters. Gel electrophoresis at different pHs and gel concentrations showed a single major band corresponding to 19S thyroglobulin in rabbits, with occasional faint bands corresponding to 12S and 27S species. The thyroglobulins of the normals differed from each other in electrophoretic pattern on sodium dodecyl sulfate (SDS)-urea gels and in composition of iodine, monosaccharides, and amino acids. Nine amino acids showed significant variation among the five thyroglobulins at the P less than 0.01 level, and only two (lysine and alanine) did not vary. The content of both sialic acid and fucose varied widely, but their sum was similar among the five samples. Thyroglobulin samples from the goiters differed from the normals and from each other in composition and in pattern on SDS-urea gels. The variability itself was more impressive than were differences in any particular component. Relative to the normals, these thyroglobulins showed increases in content of sialic acid (P less than 0.01) and lysine (P less than 0.10), and increases in the faster bands on gel electrophoresis in SDS-urea. Two goiters were from patients with the multiple hamartoma syndrome, and the only metabolic abnormality found was a low content of iodothyronine in thyroglobulin. The other two goiters also showed inadequate coupling of iodotyrosyls. In addition, one contained a soluble iodoprotein of very high molecular weight, which was immunologically identical to 19S thyroglobulin but differed in chemical composition. We conclude from the compositional data that there is not a single structure for "normal" thyroglobulin, but that multiple molecular configurations occur naturally and are compatible with adequate hormone synthesis. Extensive variations in thyroglobulin structure are frequently found with goiter, and we suggest that these may be involved in its pathogenesis.