Thyroid gland function during childhood and adolescence. Changes in serum TSH, T4, T3, thyroxine-binding globulin, reverse T3 and free T4 and T3 concentrations.

Abstract. Serum thyrotrophin (TSH), thyroxine (T 4 ), triiodothyronine (T 3 ), thyroxine-binding globulin (TBG) and reverse T 3 (rT 3 ) 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 T 4 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 T 3 levels were lower after 13.0 years than previously; serum rT 3 decreased between 11.0 and 11.9 years and rose thereafter; the calculated serum free T 4 (FT 4 ) and free T 3 (FT 3 ) concentrations had a significant rise from 14.0 to 15.9 years followed by a sharp decline; T 3 :T 4 , rT 3 :T 3 and rT 3 :T 4 ratios were constant up to 11.9 years, then a rise was seen in T 3 :T 4 and a fall in the later ratios, followed by a drop in T 3 :T 4 ratio and a sustained rise in rT 3 :T 3 and rT 3 :T 4 ratios. In boys, TSH levels were constant between 5.2 ± 0.4 and 6.6 ± 0.4 μU/ml; serum T 4 decreased with increasing age; serum TBG was constant up to 13.9 years, and had a sustained fall thereafter; serum T 3 was constant over the age range studied; serum rT 3 levels decreased up to 13.9 years and rose thereafter; FT 4 had no changes with increasing age while FT 3 , although constant up to 13.9 years, had a sustained rise afterwards; T 3 :T 4 ratio did not change with age, while rT 3 :T 3 and rT 3 :T 4 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.