There is a complex bidirectional relationship between rheumatic diseases and cancer. Certain rheumatic diseases, in particular dermatomyositis, polymyositis, rheumatoid arthritis, systemic lupus erythematosus, Sjogren’s syndrome, and systemic sclerosis, on the one hand, are associated with an increased risk of malignant neoplasms against the background of the disease, contributing to cancer due to immunological stimulation. On the other hand, it can manifest as a result of autoimmune reactions caused by primary cancer in the form of paraneoplastic syndrome. Paraneoplastic syndrome is a symptom or set of symptoms that are secondary to the primary cancer process. These symptoms can result from substances secreted by the tumor (hormones and other biologically active substances) or due to the immune reaction to tumor cells (autoimmune reactions, the formation of immune complexes, suppression of the immune system), involving various organs and systems distant from the primary focus or metastases. This paper presents a literature review about the relationship between autoimmune diseases, to a greater extent systemic lupus erythematosus, and malignant neoplasms, and a clinical case in which the manifestation of lupus nephritis coincided with the detection of papillary thyroid carcinoma.