Reevaluation of thyroid function in low birth weight infants based on free triiodothyronine and triiodothyronine.

We attempted to analyze the physiological meaning of abnormally low thyroid hormones in low birth weight (LBW) infants on the basis of cross-sectional data on free triiodothyronine (free T3) and T3 in addition to the other thyroid hormones. In LBW infants, the levels of free T3 and T3 were lower than those of normal infants, similar to the relation between free thyroxine (free T4) and T4. The ratio of free T3/T3 or free T4/T4, however, was not different between the groups. The levels of thyroid-stimulating hormone, thyroid hormone-binding globulin and reversed T3 in LBW infants were not significantly different from those of normal infants.