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

Abstract

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±0.3μg/dl, 24.4 1.1 μg/ ml, mean±SE) were significantly lower than the levels in maternal blood (12.1±0.6 μg/di, 40.3±2.3 μg/ml), whereas mean f-T₄ concentrations in cord blood (1.13±0.04 μg/dl) were significantly higher than the values in maternal blood (0.86±0.03 μg/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 1-3 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 1-3 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 1-3. This indicates that the change of activity of 5-monodeiodinase was different from that of 5'-monodeiodinase.
In the prepubertal and pubertal boys and girls, f-T₃ levels in early pubertal (Tanner stage II) boys were slightly higher, and t-T₃ levels in pubertal (Tanner stage IV) boys were lower than the values in other stages while there were no significant changes in girls. These results suggest that these changes were due to the changes of TBG concentration and/or of conversion of T₄ to T₃.