Thyroid gland function during childhood and adolescence

Abstract

: Serum thyrotrophin (TSH), thyroxine (T₄), triiodothyronine (T₃), thyroxine-binding globulin (TBG) and reverse T₃ (rT₃) were measured by radioimmunoassay in 175 girls and 187 boys aged 6.0 to 16.9 years, who were clinically healthy, and had negative serum antithyroglobulin and antimicrosomal antibodies. All the children had normal weight and height and were grouped at 12 months' intervals. In girls, TSH levels ranged between 5.3 ± 0.4 and 6.9 ± 0.5 μU/ml without significant changes with age; serum T₄ decreased up to 13.9 years and rose afterwards; serum TBG was constant up to 13.9 years, decreased subsequently and rose after 15.9 years; serum T₃ levels were lower after 13.0 years than previously; serum rT₃ decreased between 11.0 and 11.9 years and rose thereafter; the calculated serum free T₄ (FT₄) and free T₃ (FT₃) concentrations had a significant rise from 14.0 to 15.9 years followed by a sharp decline; T₃:T₄, rT₃:T₃ and rT₃:T₄ ratios were constant up to 11.9 years, then a rise was seen in T₃:T₄ and a fall in the later ratios, followed by a drop in T₃:T₄ ratio and a sustained rise in rT₃:T₃ and rT₃:T₄ ratios. In boys, TSH levels were constant between 5.2 ± 0.4 and 6.6 ± 0.4 μU/ml; serum T₄ decreased with increasing age; serum TBG was constant up to 13.9 years, and had a sustained fall thereafter; serum T₃ was constant over the age range studied; serum rT₃ levels decreased up to 13.9 years and rose thereafter; FT₄ had no changes with increasing age while FT₃, although constant up to 13.9 years, had a sustained rise afterwards; T₃:T₄ ratio did not change with age, while rT₃:T₃ and rT₃:T₄ ratios, although constant up to 13.9 years, showed a tendency toward a sustained rise thereafter. These sex-different variations in serum thyroid hormone concentrations might be related to the fact that girls mature at an earlier chronological age than boys and may represent a partial response of the body to the qualitatively and quantitatively different energy needs in girls as compared with boys, consecutive to the differences in body composition first appearing at puberty.