Determination of Replacement and Suppressive Doses of Thyroxine

Abstract

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 μg/kg lean body mass or 1 – 2 μg/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 μg/kg lean body mass or 2 μg/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 μg/kg lean body mass or 1 – 2 μg/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 μg/kg lean body mass or 2 μg/kg body weight, whereas approximately 30% of non-toxic nodular goitre patients required less than 3 μg/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.