SUMMARY In most cases of Graves' disease the level of iodotyrosines in the plasma is above normal. With a view to ascertaining the cause of this phenomenon simultaneous determinations were made of the iodotyrosine and long-acting thyroid stimulator (LATS) levels in the plasma and of the titre of thyroid antibodies. It was found that high plasma concentrations of iodotyrosines in exophthalmic goitre were mainly due to auto-immune thyroiditis, but that LATS may also contribute to the phenomenon.