A Study on the Changes with Age in Thyroid Function during Infancy, Childhood and Adolescence

Serum total thyroxine (t-T4), free thyroxine (f-T4), total triiodothyronine (t-T3), free triiodothyronine (f-T3), reverse triiodothyronine (r-T3) and thyroxine-binding globulin (TBG) levels were measured by radioimmunoassay in pregnants, infants, children and adolescents. The mean t-T4 and TBG concentrations in cord blood (9.0 +/- 0.3 micrograms/dl, 24.4 +/- 1.1 micrograms/ml, mean +/- SE) were significantly lower than the levels in maternal blood (12.1 +/- 0.6 micrograms/dl, 40.3 +/- 2.3 micrograms/ml), whereas mean f-T4 concentrations in cord blood (1.13 +/- 0.04 micrograms/dl) were significantly higher than the values in maternal blood (0.86 +/- 0.03 micrograms/ml). The mean t-T3 and f-T3 concentrations in cord blood (48 +/- 2 ng/dl, 1.1 +/- 0.1 pg/ml) were much lower than the levels in maternal blood (136 +/- 5 ng/dl, 2.6 +/- 0.1 pg/ml). The r-T3 concentrations in cord blood (192 +/- 4 ng/dl) were markedly higher than the maternal levels (46.6 +/- 3.7 ng/dl). These results suggest that in the fetus the conversion of T4 to r-T3 was greater than that of T4 to T3 due to a greater ratio of activity of 5-monodeiodinase to that of 5'-monodeiodinase. In relation to the changes in thyroid function after birth, t-T4 and f-T4 values showed parallel changes while t-T3 and f-T3 levels increased rapidly during the first month and t-T3 levels remained unchanged between 1 and 11 months, whereas f-T3 levels and f-T3/t-T3 ratio increased gradually during the same time. T-T3 levels in children 1 to 15 years of age were slightly lower than the levels in infants 1 to 11 months. R-T3 values decreased progressively from 5 days to 1 approximately 3 months and continued to decline until 4 years of age. Values between 4 and 15 years were not changed. T-T3/t-T4 ratio increased markedly 1 approximately 3 months after birth. This indicates that the activity of 5'-monodeiodinase was elevated rapidly after birth and suggests that the rapid increase of T3 levels after birth was due to enhanced enzymatic activity as well as higher TSH levels. R-T3/t-T4 ratio decreased rapidly after birth and continued to decrease during years 1 approximately 3. This indicates that the change of activity of 5-monodeiodinase was different from that of 5'-monodeiodinase.(ABSTRACT TRUNCATED AT 400 WORDS)