Differentiation of thyrotoxicosis induced by thyroid destruction from Graves' disease.

Thyroid function was tested in untreated patients with Graves' disease or thyrotoxic subacute thyroiditis, and in patients with autoimmune thyroiditis who showed postpartum or spontaneous transient thyrotoxicosis. The serum triiodothyronine/thyroxine ratio (T3/T4) was greater than 20 ng/microgram in Graves' disease but less than 20 in all patients with subacute thyroiditis and 9 of 11 patients with autoimmune thyroiditis. Thus, like radioactive iodine uptake, the serum T3/T4 ratio is useful for differentiating destruction-induced thyrotoxicosis from the stimulation-induced hyperthyroidism of Graves' disease.