Serum and urine thyroid hormone levels in healthy preterm and small for date infants on the first and fifth day of life.

Serum levels of thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (rT3), thyroxine binding globulin (TBG) and thyrotropin (TSH) were determined on the first and the fifth day of life in three groups of healthy premature infants (Group I: 26-32 weeks of gestation, Group II: 33-36 weeks, and Group III: full term but small for date). Urine T4 and T3 were also determined. T4 and T3 serum concentrations increased from Group I to Group III, but were similar on day 1 and 5 in each group. rT3 was higher in Group I on day 1. The low concentrations of T4 were not dependent on the TBG levels. TSH levels were higher on day 1 than on day 5, but the hormonal feed back mechanism was not effective on day 1. Hormonal urine excretions were similar on day 1 and 5, except a higher T4 excretion in Group III on day 5. These results were related to the maturation steps in healthy premature infants where gestational age seemed more important than body weight in respect to some thyroid hormone parameters.