We examined the relationship of thyroid suppressibility to long-acting thyroid stimulator (LATS) levels in 14 thyrotoxic patients on long-term treatment with antithyroid drugs (mean duration of treatment, 11.9 months). Twenty-minute131 I thyroid uptake and LATS levels before and after liothyronine sodium administration were measured before and at one- to three-month intervals during antithyroid drug treatment. Eight patients developed thyroid suppressibility during therapy; of these, three had detectable LATS levels prior to treatment and there was no decline in LATS levels with treatment and restoration of suppressibility. Of six patients whose thyroid uptake remained nonsuppressible during long-term treatment (mean, 11.1 months), LATS was undetectable in both the serum and concentrated immunoglobulin G (IgG) fraction of four. These studies suggest that in exophthalmic goiter (Graves' disease), thyroid nonsuppressibility may reflect an intrinsic thyroid abnormality rather than the presence of circulating LATS.