Autoimmune thyroid disease associated to Sjögren syndrome (SS) may be defined by serological and functional abnormalities. We studied the prevalence of serum anti-thyroid autoantibodies and the development of thyroid functional defects by a follow-up investigation in patients with primary (pSS) and secondary SS (sSS). In keeping with previous literature data, our findings confirmed an increased prevalence of thyroid autoimmune phenomena in the whole series of patients with SS. In addition, evidence was provided for a greater incidence of serological and functional thyroid abnormalities in pSS when compared to sSS. Follow-up study also showed that the rate of thyroid autoimmune disease development was higher in pSS than in sSS. The appearance of circulating anti-thyroid antibodies was particularly frequent in patients with preexisting antibodies against extractable nuclear antigens (anti-ENA). These data indicate the need for a careful monitoring of thyroid function in SS patients, with particular regard to those with pSS and positive anti-ENA tests.