Tablets containing 50mug. of thyroxine and 15mug. of triiodothyronine have been used to treat patients with hypothyroidism of varied aetiology. Following total thyroidectomy a daily dose of three tablets-that is, 150mug. of thyroxine and 45mug. of triiodothyronine-maintained patients in a euthyroid state and produced a protein-bound iodine level within the normal range. The preparation is preferable to dried thyroid because of uniformity of activity and speed of action, and when the patient is clinically euthyroid the P.B.I. falls within the normal range.