Thyroid hormone concentrations and free thyroxine status in neonatal venous and capillary serum pairs

The levels of thyroid hormones in paired venous and capillary serum samples taken from 68 healthy full-term neonates were determined by radioimmunoassay. Compared with capillary values, total thyroxine (T4), thyrotropin (TSH) and thyroxine-binding globulin (TBG) each showed a significant decrease in venous serum. The mean venous and capillary concentrations of tri-iodothyronine (T3), reverse tri-iodothyronine (rT3) and free thyroxine (fT4) were not significantly different; however, in each case the fitted linear relationship suggested that venous and capillary values were not concordant (P less than 0.02, P less than 0.01, P less than 0.05 respectively). Both the T3 uptake test and the free thyroxine index (FTI) were significantly higher in venous serum, while the ratios of T4 to TBG in paired samples were equivalent. These results suggest that consistent sample collection and assay methods should be applied when assessing neonatal thyroid status, particularly if comparison of results is intended.