Among more than 7000 adult patients with suspected dysthyroidism in who radioimmune assays were performed, the authors have selected 21 patients with high TSH hypothyroidism who had assays before and after replacement therapy with various preparations prescribed by their G.P. The results in this group of 61 treatments of at least 3 weeks duration were as follows: (1) under thyroid extract or thyroglobulin (21 treatments), the condition persisted in the majority of patients; (2) under thyroxine (31 treatments), the thyroid function returned to normal with a once-a-day dose of 150 mcg/-T4; (3) under triiodothyronine (7 treatments), none of the patients reverted to euthyroidism. Levothyroxine therapy therefore appears to be the most effective and cheapest way of providing optimal replacement therapy in patients with hypothyroidism. The daily dose of about 150 mcg can easily be adjusted according to the results of occasional hormonal assays.