We investigated whether there was a significant increase in thyroid autoimmunity, and disorders in patients with rheumatic diseases (RDs). We enrolled 201 patients with RDs (41 with ankylosing spondylitis, 15 with systemic lupus erythematosus, 80 with rheumatoid arthritis [RA], 65 with familial Mediterranean fever), and 122 healthy controls. Serum levels of thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), free thyroxine (fT4), C-reactive protein, and thyroid autoantibodies (anti-thyroglobulin and anti-thyroid peroxidase) were measured in all participants. There were no significant differences between the ages of the patients and controls. The mean TSH values of the patients with RDs and the controls were 3.1 ± 2.68 mIU/L and 1.9 ± 0.83 mIU/L, respectively (P = 0.004). The mean fT4 value of the patients with RDs was 1.43 ± 0.67 ng/dL whereas that of the controls was 1.58 ± 0.68 ng/dL (P <0.001). Subclinical hypothyroidism was detected in 24 patients with RDs. Thyroid antibodies were detected in 16 of 201 (8%) patients with RDs. Three of these patients had subclinical hypothyroidism, while the others were euthyroid. Thyroid autoantibodies were significantly higher in patients with RDs (P <0.001). Additionally, thyroid disorders were observed more frequently in patients with RDs than in the healthy controls. Based on our findings, we recommend that thyroid function tests should better be included in the clinical evaluation of patients with RDs.