Intra-individual variation of serum thyroxin and triiodothyronine in pregnancy

The mean concentrations of triiodothyronine (T3) and thyroxin (T4) in serum were increased in pregnancy, the increases for individuals remaining stable for week 16 to week 40 of gestation. For this period biological intra-individual variations of T3 and T4 in serum were estimated and compared with those of non-pregnant women. The average biological intra-individual CVs for T3 and T4 were of the same order for pregnant and non-pregnant women (6.9-8.4%). The ratios of the biological intra-individual CVs to the biological group CVs were 0.5 to 0.6. Individual values were normally distributed. There was no increase of the intra-individual variation with the lapse of time between two consecutively observed values. The estimated average biological intra-individual CVs were used to derive decision-making criteria in monitoring thyroid function during the 2nd and 3rd trimester of gestation.