Introduction: Autoimmune diseases, including systemic lupus erythematosus (SLE), are associated with an increased risk of developing malignant tumors. Thyroid cancer is the most common cancer of the endocrine system. It has been shown that SLE increases the risk of thyroid cancer by approximately twofold, especially in young women. Possible causes of this association include the relationship between autoimmunity and carcinogenesis, high disease activity, the impact of immunosuppressive treatment and the appearance of connective tissue disease symptoms during the neoplastic process (paraneoplastic syndrome).

Case report: We present the case of a 40-year-old female SLE  patient  who has been treated with multiple immunosuppressive drugs, including glucocorticosteroids (gcs), hydroxychloroquine, azathioprine, methotrexate, mycophenolate mofetil (MMF), and epratuzumab (anti-CD22). Complete remission of the disease was not achieved. The clinical presentation was characterised by skin changes indicative of subacute cutaneous lupus erythematosus, as well as vasculitis, arthritis, weight loss and haematological abnormalities.

In 2024, an ultrasound examination of the thyroid gland revealed a focal lesion in the right lobe and a thyroid biopsy raised suspicion of papillary carcinoma. A total thyroidectomy was performed, followed by the administration of an ablative dose of J-131 and suppressive doses of L-thyroxine. Several weeks after surgery, a significant improvement in the skin lesions was observed. Symptoms of SLE remission persist to date.

In summary, any systemic connective tissue disease, including SLE, requires oncological vigilance. The coexistence of SLE and differentiated thyroid cancer is rare. Periodic testing for antithyroid antibodies and ultrasound examinations of the thyroid gland are recommended for all patients with SLE.