Graves' disease (GD) and Hashimoto's thyroiditis (HT) are autoimmune processes often associated with hyperthyroidism and hypothyroidism, respectively. Despite their diverging clinical presentations, immune activation drives both diseases and results in connective tissue accumulation of the non-sulfated glycosaminoglycan, hyaluronan. The hydrophilic property of hyaluronan contributes to the pathogenesis of thyroid associated ophthalmopathy, GD-associated dermopathy and hypothyroid myxedema. Whether hyaluronan accumulates in the thyroid and plays a role in goiter formation in GD and HT remains unknown. We report here that levels of hyaluronan are increased in thyroid tissue from individuals with both diseases compared with glands uninvolved with autoimmune disorders. The transcript encoding hyaluronan synthase (HAS)-3, one of three mammalian HAS isoforms, was detected in thyroid tissue. Isolated thyrocytes in primary culture express all three HAS isoforms when treated with IL-1. Thyrocytes and thyroid fibroblasts produce hyaluronan, under basal culture conditions and IL-1 enhances levels of this molecule in both cell-types. On a per cell basis, fibroblasts produce more hyaluronan than do thyrocytes under basal conditions and following cytokine treatment. Synthesis in thyrocytes can also be altered by increasing serum concentration in the medium and by modifying culture density. Our findings suggest that hyaluronan accumulation in thyroid tissue might derive from thyrocytes and fibroblasts. Moreover, this glycosaminoglycan becomes more abundant as a consequence of autoimmune disease. It may therefore contribute to increased thyroid volume in GD and HT. Coupled with the newly identified influence exerted by hyaluronan on immunocompetent cells, our findings represent potentially important insights into the pathogenesis of autoimmune thyroid diseases.