Acute changes in thyroid function tests following ingestion of thyroxine.

The timing of blood sample collection in relation to ingestion of thyroxine has been thought to be of no consequence in the assessment of patients receiving thyroxine medication. We have investigated changes in serum thyroid hormones after oral ingestion of thyroxine. Therapeutic doses of thyroxine (100-300 micrograms) were given to five normal, euthyroid subjects and eleven patients receiving long-term thyroxine-replacement medication. Blood samples were collected prior to and following ingestion of thyroxine. A significant increase in total thyroxine (T4), free thyroxine index (FTI) and free T4 (FT4) concentration was observed at all doses. No significant change was observed in 3,5,3'-triiodothyronine, 3,3',5'-triiodothyronine or thyrotrophin concentrations. Although T4, FTI and FT4 were significantly elevated 1 h after ingestion of all doses of thyroxine and remained elevated for at least 6 h, supranormal values were observed only after ingestion of the highest dose of thyroxine. The levels of T4, FTI and FT4 in patients receiving thyroxine should be interpreted in relation to the time of thyroxine administration. Standardisation of blood collection in patients receiving thyroxine replacement would be desirable.