1 µl of each cDNA was used for measurement of the copy number. The conditions for the TaqMan PCR were as follows: 95˚C for 10 min, followed by 40 cycles of 95˚C for 15 seconds and 60˚C for 1 min. Recombinant pGEM T-vectors (Promega, Tokyo, Japan) containing TG or GAPDH cDNA were constructed by PCR cloning with the same set of primers used in the TaqMan PCR and were used as standard samples. The expression levels of TG mRNA were calculated by dividing the copy number of TG mRNA by that of GAPDH mRNA. Intraassay variabilities were 35% and 24% at 10 and 100 ng l -1 thyroid total RNA, respectively. Interassay variabilities across a 1-month period were 38% and 28% at 10 and 100 ng/L thyroid total RNA, respectively.