Reverse tri-iodothyronine to tri-iodothyronine ratio and gestational age

The biologic significance of reverse tri-iodothyronine is not yet identified. Cord blood values reflect the fetal thyroid state, with rT3 excess and tri-iodothyronine deficiency. In an attempt to correlate thyroid function and gestational age, T3 and rT3 concentrations were measured by radioimmunoassay on 64 cord blood samples. Infant gestational ages ranged from 29 to 42 weeks; birth weights from 650 to 3,870 gm. The results show a significant positive correlation between gestational age and T3 and a significant negative correlation between gestational age and rT3. The rT3/T3 offers the best correlation.