Our objectives were to investigate thyroid abnormalities and autoimmunity in 120 patients affected by fibromyalgia (FM) and to study their relationships with clinical data and symptoms. Thyroid assessment by means of antithyroglobulin antibodies, antithyroid peroxidase antibodies, free triiodo–thyronine, free thyroxine, and thyroid stimulating hormone analyses was carried out. The clinical parameters “Fibromyalgia Impact Questionnaire”, pain, tender points, fatigue, and other symptoms, and the presence of depression or anxiety disorders were evaluated. The basal thyroid hormone levels of FM patients were in the normal range, while 41% of the patients had at least one thyroid antibody. Patients with thyroid autoimmunity showed a higher percentage of dry eyes, burning, or pain with urination, allodynia, blurred vision, and sore throat. Correlations found between thyroid autoimmunity and age or with the presence of depression or anxiety disorders were not significant. However, in the cohort of post-menopausal patients, the frequency of thyroid autoimmunity was higher with respect to pre-menopausal patients. In conclusion, autoimmune thyroiditis is present in an elevated percentage of FM patients, and it has been associated with the presence of typical symptoms of the disease.