1. A study was made of the activity of the enzyme thyroid peroxidase and of the concentration, carbohydrate composition and the degree of iodination of thyroglobulin in the thyroid glands of 60 patients with non-endemic non-toxic goitre in the nodular phase and in those of 25 control subjects. 2. Thyroid peroxidase activity was determined by the guaiacol assay and was significantly higher in patients with non-endemic non-toxic goitre than in control subjects (3.60 ± 2.51 and 2.07 ± 1.08 μmol of guaiacol oxidized min −1 g −1 of tissue, respectively; ranges 0.16–10.57 and 0.52–4.85 μmol of guaiacol oxidized min −1 g −1 of tissue, respectively; P <0.05). 3. Thyroglobulin was purified by precipitation with ammonium sulphate and Sephadex G-200 gel filtration. Two protein peaks were obtained which were identified as thyroglobulin and measured by radioimmunoassay. The concentration of thyroglobulin in the first peak was 98.94 (SD 84.87, range 0.60–455.54) mg/g of tissue for the patients with non-endemic, non-toxic goitre and 51.41 (SD 28.34, range 14.99–106.39) mg/g of tissue for the control subjects ( P <0.01). The second peak showed 1.26 (SD 1.27, range 0.09–6.50) mg of thyroglobulin/mg of tissue for the group with non-endemic non-toxic goitre and 0.51 (SD 0.25, range 0.15–0.98 mg of thyroglobulin/mg of tissue) for the control subjects ( P <0.01). 4. The carbohydrate composition of thyroglobulin was determined by acid hydrolysis and colorimetry, evaluating the levels of hexoses, hexosamines and sialic acid. The degree of glycosylation was significantly higher in thyroglobulin from patients with non-endemic non-toxic goitre (mean 16.46%, SD 5.68%, range 8.33–37.3% for the first peak, and mean 16.11%, SD 4.85%, range 5.30–33.10% for the second peak) than that from control subjects (mean 9.62%, SD 1.23%, range 7.56–12.02% for the first peak, and mean 8.68%, SD 0.99%, range 7.24–11.09% for the second peak). The values for each of the carbohydrates were significantly higher in the thyroglobulin from patients with non-endemic non-toxic goitre than in the control subjects, and there was also a positive correlation between them, except for the hexoses and hexosamines in the second peak of the thyroglobulin from the control subjects. 5. The thyroglobulin of the group with non-endemic non-toxic goitre was significantly less iodinated (mean 0.055%, SD 0.038%, range 0.021–0.161% in the first peak, and mean 0.30%, SD 0.22%, range 0.10–0.93% in the second peak) than that of the control subjects (mean 0.32%, SD 0.16%, range 0.07–0.64% in the first peak, and mean 0.77%, SD 0.17%, range 0.37–1.27% in the second peak). 6. A significant negative correlation was observed between the degree of thyroglobulin iodination and thyroid peroxidase activity in control subjects; however, this was not found in patients with non-endemic non-toxic goitre. The mean level of peroxidase activity in the latter group was 26.71% above what would be expected for the mean degree of thyroglobulin iodination found, indicating that other additional factors may govern the increase in enzymic activity. 7. Such findings point to the existence of an alteration in thyroglobulin that may decrease the efficiency of the protein for normal hormone synthesis, and this may be one of the reasons for the development and/or maintenance of non-endemic non-toxic goitre.