Thyroid gland function during childhood and adolescence

Abstract

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.