Suppression daily doses of thyroxine (T4) were determined and the daily amounts of T4 required to replace T4 were established in 217 hypothyroid patients. Patients with Hashimoto's thyroiditis treated daily with 2-3 micrograms/kg lean body mass or 1-2 micrograms/kg body weight T4 had normal serum thyrotrophin (TSH) concentrations, normal response to TSH-releasing hormone (TRH) and normal systolic time intervals but doses higher than 3 micrograms/kg lean body mass or 2 micrograms/kg body weight decreased serum TSH concentrations, with no response to TRH and systolic time intervals typical of hyperthyroidism. In 13/32 (41%) hypothyroid patients with Graves' disease following 131I and/or surgery, the daily T4 replacement dose was similar to that in Hashimoto's thyroiditis patients but in 12 (38%) patients daily doses of 2-3 micrograms/kg lean body mass or 1-2 micrograms/kg body weight T4 increased serum T4 and suppressed TSH levels, and in six (9%) lower doses were required to control hypothyroidism. The T4 suppression dose for patients with thyroid cancer was more than 3 micrograms/kg lean body mass or 2 micrograms/kg body weight, whereas approximately 30% of non-toxic nodular goitre patients required less than 3 micrograms/kg lean body mass. It is concluded that replacement or suppression doses of T4 should be individually determined and that different criteria should be applied for their calculation depending on the thyroid abnormality.