To evaluate in a cohort of 100 consecutive patients affected by primary Sjogren's syndrome (pSS) the incidence of Hashimoto thyroiditis (HT) and to compare the clinical features and the laboratory parameters of patients affected by pSS with and without concomitant HT. In 100 consecutive patients affected by pSS, the occurrence of other autoimmune diseases was recorded and a full examination of thyroid function obtained. HT was associated with pSS in 27 cases. The comparison between pSS cases with and without HT showed that only patients with isolated pSS had low C4 level [p = 0.032, OR (IC 95 %) 230 (13.13-4,046)]. In addition, only patients affected by pSS without HT had evidence of cryoglobulins, cutaneous vasculitis with palpable purpura, peripheral neuropathy, and development of lymphoma, although all these manifestations were observed in a 4.1-8.2 % of the cases, without reaching statistical significance. The association of HT in patients suffering from pSS defines a subset of patients with milder disease and normal C4 levels.