TWENTY‐FOUR HOUR VARIATIONS OF THYROID HORMONES AND THYROTROPHIN CONCENTRATIONS IN HYPOTHYROID INFANTS TREATED WITH L‐THYROXINE

SUMMARY The 24 hour plasma profile of T4, FT4, T3 and TSH was measured in a group of infants treated for congenital hypothyroidism with an oral aqueous preparation of L‐T4 (5‐7.5 μ g/kg) given as a single dose at 0800 h. Mean basal plasma T4 was 8.4 ± 2.0 μ g/dl (mean ± SD) and remained constant during the study while mean serum FT4 increased significantly from 1.64 ± 0.50 ng/dl to a maximum of 2.08 ± 0.63 ng/dl 4 h after medication. Plasma T3 decreased significantly from 241 ± 48 ng/dl to 202 ± 36 ng/dl 6 h after drug administration. Finally, plasma TSH decreased significantly from a mean of 24.2 ± 18.8 μU/ml to 14.8 ± 10.5 μU/ml 6 h later. Therefore during treatment of congenital hypothyroidism with L‐T4 no specific schedule for T4 sampling is required. However, TSH determination 4 to 8 h after medication could slightly underestimate mean levels. In addition, any attempt to monitor treatment by FT4 determination should take into consideration variations of plasma values in the hours following L‐T4 absorption.