Effect of Triiodothyronine Administration on Serum PBI in Hypothyroid Patients Maintained on Constant Doses of Thyroxine

Thirteen patients with primary hypothyroidism were given oral thyroxine (T4) alone. Triiodothyronine (T3) was then administered orally in increasing amounts, while the dose of T4 was kept constant. Increasing doses of T3 were accompanied by a fall in the serum PBI concentration in all patients. There was a slight rise in the mean BMR for the group, but other laboratory indices of thyroid function did not vary in any consistent fashion. The inadequacy of current methods for assessing accuracy of replacement doses of thyroid hormones in patients, and probably other endocrine replacement medications as well, is emphasized.