Variation in serum levels of T3, T4, FT4 and TSH during thyroxine replacement therapy

Serum levels of T3, T4, FT4 and TSH were determined in 9 euthyroid patients during 6 h after intake of morning thyroxine replacement medication. Four hypothyroid patients were studied in the same way after the intake of 50 micrograms thyroxine. In the euthyroid patients the FT4 level increased by 31%, while the increase of T4 was 16% and the increase of T3 11%. In the hypothyroid patients levels of T3, T4 and FT4 did not change after ingestion of 50 micrograms thyroxine, while the TSH level was decreased by 30%. It is concluded that during thyroxine replacement therapy hormone analysis must be interpreted in relation to the time of thyroxine administration.