BackgroundRecent studies have revealed that thyroid and autoimmune diseases may be associated with sporadic moyamoya disease. However, whether routine screening serum tests to detect these underlying diseases are useful or not remains unclear. MethodsWe retrospectively evaluated 459 patients with moyamoya disease but without previous history of thyroid or autoimmune diseases who underwent the screening serum tests targeting thyroid and autoimmune diseases from 2016 to 2023 in our institute. The number of patients who were diagnosed as thyroid or autoimmune diseases after these tests were investigated. ResultsAmong the patients who were screened, 237 (42.6 %) patients had abnormal results for some factors, such as thyroid hormones or autoantibodies. After consultation with endocrinologists or rheumatologists, 27 (5.9 %) patients were newly diagnosed with thyroid diseases, including six (1.3 %) patients with Graves’ disease, 19 (4.1 %) patients with Hashimoto thyroiditis and two (0.4 %) patients with other thyroid diseases; however, none of the patients were diagnosed with nonthyroidal autoimmune diseases, such as Sjogren’s syndrome, antiphospholipid syndrome, or rheumatoid arthritis, listed as moyamoya-related diseases and targeted by our screening serum tests. Patients with newly diagnosed underlying diseases were more likely to be female compared to patients without new diagnosis (96.3 % vs. 72.2 %, p = 0.03). ConclusionRoutine thyroid-related serum screening may be clinically meaningful in patients with moyamoya disease to detect occult thyroid diseases, especially in female patients. However, routine serum screening tests targeting other autoimmune diseases are not recommended unless the patients have equivalent symptoms.