[Effect of various sampling condition on the measurement of circulating TSH and free thyroxine]

We report here the effect of various sampling and storage condition on the measurement of circulating TSH and free thyroxine (FT4). Blood samples were obtained from five normal subjects using four kinds of sampling tubes; plane glass tube, glass tube containing serum separator, and glass tube containing EDTA-2Na or heparin-sodium. We stored whole blood, serum or plasma at low temperature (4-8 degrees C) or at room temperature (20-25 degrees C) from one hour to thirteen days. Both values of EDTA-plasma TSH and FT4 were significantly lower than those of serum samples, EDTA-plasma TSH value was increased gradually during storage at room temperature but not at low temperature. Serum or plasma FT4 values were decreased during storage at room temperature but not at low temperature. In conclusion, circulating TSH and FT4 values are influenced by different sampling tubes and, serum or plasma TSH and FT4 are not always stable under the storage condition at room temperature. Serum samples obtained by using plane glass tube and kept at low temperature give us more reliable values of TSH and FT4.