Thyroid hormone production by metastases of differentiated thyroid carcinoma is very rare and its pathogenesis is still unknown. The aim of this study was to present some clinical and demographic evidence that thyroid hormone-producing metastases of differentiated thyroid carcinoma are related to environmental factors, probably iodine deficiency. A cross-sectional study was performed on thirty-five patients with distant metastases, identified in a group of 125 patients with differentiated thyroid carcinoma previously submitted to total or near total thyroidectomy. In 6 patients (5 females, 1 male; age range, 50 to 64 yr) we had evidence that the metastases were actively producing thyroid hormones and in 29 patients (21 females, 8 males; age range 8 to 84 yr) the metastases were considered to be nonthyroid hormone-producing. Serum levels of T3, T4, and thyroglobulin were measured by RIA, TSH by IRMA, and 131I whole-body scintigraphy was performed 72 h after 187 Mbq of 131I. All patients with metastases producing thyroid hormones presented a pure follicular thyroid carcinoma. They also differed from patients with nonproducing metastases in the frequent presence of goiter of long duration as the first clinical manifestation of thyroid disease (p < 0.01), and a higher proportion of patients coming from an iodine deficient area (5/6 vs. 6/29, p < 0.05). In these patients the serum thyroglobulin levels tended to be higher (p = 0.069) as compared with the nonproducing metastases group. In conclusion, a late diagnosis of follicular carcinoma in patients with long-standing multinodular goiter allowed the development of well differentiated and bulky metastases retaining the ability to produce thyroid hormones.