Changes in plasma thyroxine (T4), triiodothyronine (T3), free thyroxine index (FT4I) and thyroid stimulating hormone were studied in 100 patients with Graves' disease treated with carbimazole. During therapy plasma T3 concentrations were disproportionately high compared to those of T4, the T4 : T3 ratio was low, and many patients were clinically euthyroid with a normal plasma T3 but low T4 concentration. Although there was considerable individual variation in response, the order of response was always the same with plasma T4 falling to normal or low levels before T3. Plasma T3 was the best indicator of clinical status and the best predictor of the impending change; additional information of changes in thyroid status was obtained from plasma T4 and FT4I estimation, especially when these were followed sequentially. Single measurements of T4 or FT4I only are not recommended for assessing thyroid function during carbimazole therapy.