Abnormalities in the thyroid function and thyroid autoantibodies have been frequently described in patients with autoimmune diseases but seldom in antiphospholipid syndrome patients. In order to determine the prevalence of thyroid function and autoimmune abnormalities, we compared serum thyrotropin (TSH, serum free thyroxine (T4) levels, thyroid antithyroglobulin (TgAb) and antithyroperoxidase (TPOAb) levels of 25 patients with systemic sclerosis, 25 patients with rheumatoid arthritis and 13 patients with antiphospholipid syndrome to a control group of 113 healthy individuals. Evaluation included a thorough clinical examination with particular attention to thyroid disease and a serologic immune profile including rheumatoid factor, antinuclear and anticardiolipin antibody measurements. Subclinical hypothyroidism (4.2<TSH<10 mU/L) was diagnosed in five patients (8%), and subclinical hyperthyroidism (undetectable<TSH<0.34 mU/L) in four patients (6%). Anti-thyroglobulin (TgAb) and/or anti-thyroperoxidase (tPOAb) antibodies were present in 21/63 (33%) of our patients: 13/25 (52%) of the systemic sclerosis cases, 8/25 (32%) of the rheumatoid arthritis patients, but 0 (0/13) of the antiphospholipid syndrome patients. In conclusion, our data confirm a high prevalence of silent autoimmune thyroid diseases in association with systemic sclerosis and rheumatoid arthritis (p<0.02), but not with antiphospholipid syndrome. Elevated antibody titres may reflect an epiphenomenon of the underlying autoimmune disorders and play an additive role in the development of the euthyroid sick syndrome in these patients, but our data suggest that the antiphospholipid syndrome presents a different pattern of response. Subclinical thyroid diseases should be considered when evaluating patients with autoimmune diseases.