Thyroid-stimulating autoantibodies (TSaab) react with the thyrotrophin receptor of the thyroid cells to cause the hyperthyroidism of Graves' disease. Studies using thyroid receptors from various animal species have demonstrated the existence of a fine variation in the specificity of TSaab from various patients. Such variation provides an explanation for the clinical variety seen in some autoimmune diseases and also for the occurrence of inconstant complications such as exophthalmos, chorea, diabetic retinopathy and diabetic nephropathy, which may be based on cross-tissue reactivity.