Effects of weekly thyroxine administration on serum thyroxine, 3,5,3'-triiodothyronine, thyrotropin, and the thyrotropin response to thyrotropin-releasing hormone.

Abstract

Once weekly administration of T4 to normal subjects was used to suppress pituitary TSH secretion. In four euthyroid subjects, 1.5 mg T4/week were given for 4 weeks. Seven days after each dose of T4, serum T4 was not significantly different from the baseline value, although at two intervals, serum Ta was modestly but significantly increased over the basal value. Serum TSH was suppressed to less than 0.5 μU/;ml throughout the period of T4 administration. A bolus of 500 μg TRH 3 weeks after the last dose of T4 resulted in TSH release which was identical in quantity to that found before T4 administration. A TRH infusion of 1 μg/;min for 7 h on the next day resulted in an increase in TSH to 6 μU/ml, where it remained throughout the period of the infusion. Serum T3 concentrations increased both after the 500-μg TRH test and after the 7-h infusion. Mean serum T4 was 1.3-1.6 μg/dl higher during the sixth and seventh hours of infusion than at the start of the infusion, and serum T3 increased from 135 to 205 ng/dl....