Knowledge of plasma variation of thyroid parameters is important for blood sampling during evaluation of 1-T4 treatment. 11 children, aged 2 to 12 months, treated with a single oral dose of 1-T4 at 8 am (5 to 8 μg/kg) were studied for 24h. TSH was expressed as % of basal 8 am values (m ± SD).No significant variation was observed for plasma T4 or T3. A significant plasma TSH decrease was shown 4 to 8h after drug administration. Preliminary data on serum FT4 indicate a significant increase at + 2 and + 4h (n = 6).In conclusion : due to lack of plasma T4 variations, there is no special timing for plasma sampling and T4 determination. The transient TSH decrease may underestimate plasma value if blood is taken 4 to 8h after medication. This may be due to FT4 elevation related to 1-T4 absorption.
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