Factors Affecting Suppression of Endogenous Thyrotropin Secretion by Thyroxine Treatment: Retrospective Analysis in Athyreotic and Goitrous Patients*

Factors affecting TSH suppression by l-T4 administration were retrospectively evaluated in 452 patients: 180 who were athyreotic after total thyroidectomy and remnant radioiodine ablation for differentiated thyroid carcinoma and 272 with nontoxic diffuse or nodular goiter. All patients were considered clinically euthyroid. TSH secretion was assessed by vi TRH stimulation testing. The T4 dose associated with an undetectable basal serum TSH level and no increase in serum TSH after TRH administration (suppressive dose) averaged 2.7 ± 0.4 (sd) μg/kg body weight (BW)/day in athyreotic patients and 2.1 ± 0.3 μg/kg BW/day in goitrous patients (P < 0.001). The 25th–75th percentile intervals were 2.5–2.9 μg/kg BW/day for athyreotic patients and 1.9–2.3 μg/kg BW/day for goitrous patients. The suppressive dose of T4 was dependent in both groups on patient age, younger patients needing higher doses than older patients. The duration of treatment also proved to be an important parameter, since in both groups the percen...