Serum level of type IV collagen was measured in 104 patients with various thyroid disease, and the relationship between its level and thyroid hormone level was examined. The type IV collagen was measured by the method of one step sandwich enzyme immunoassay (EIA) using two distinct monoclonal antibodies recognized triple-helical (TH) domain and 7-S domain, respectively. The serum level of type IV collagen was significantly high in the hyperthyroid patients compared with that in normal controls, and a significant positive correlation was found between its value and thyroid hormone levels (T3, T4, FT3, FT4). The elevated level of type IV collagen in hyperthyroid patients was decreased to normal level, when they became to euthyroid after antithyroid drug therapy for hyperthyroidism. The serum level of type IV collagen was in normal range in hypothyroid patients, but the value was increased to high normal range after T4-replacement therapy for hypothyroidism. This evidence indicates that the serum level of type IV collagen is closely related to thyroid hormone level in patient with various thyroid disease. Type IV collagen concentration might be one of the useful variables for evaluating the thyroid function, although its mechanism is not elucidated.