The ratio of serum free triiodothyronine to free thyroxine in children: a retrospective database survey of healthy short individuals and patients with severe thyroid hypoplasia or central hypothyroidism

BACKGROUND: The ratio of serum free triiodothyronine (FT3) to free thyroxine (FT4) has been shown to be constant in healthy adults. However, this ratio has been found to be decreased in athyreotic adult patients on levothyroxine (L-T4) supplementation. In order to better evaluate thyroid-related pathologies in children as well as to establish a reference range, we investigated the FT3/FT4 ratio in a pediatric population. Furthermore, we evaluated this ratio in children with congenital hypothyroidism as well as those with central hypothyroidism. METHODS: A reference range for the FT3/FT4 ratio was obtained from 129 Japanese children (3–17 y) with idiopathic short stature who were designated as the ‘Control’ group. Patients with congenital hypothyroidism due to athyreosis or severe thyroid hypoplasia (designated as ‘A/Hypoplasia’), as well as patients with central hypothyroidism (‘Central’), were recruited from the institutional database. For each group, the mean FT3/FT4 ratio was obtained. RESULTS: In the Control group, the FT3/FT4 ratio was 3.03 ± 0.38 10(−2) pg/ng (mean ± standard deviation) with no age or gender differences. A/Hypoplasia patients showed a significantly decreased mean FT3/FT4 ratio (2.17 ± 0.33, P < 0.001) compared to Control patients, with decreased FT3 and elevated FT4 levels. The Central group also showed a significantly decreased FT3/FT4 ratio (2.55 ± 0.45, P < 0.001) compared to the Control group, with decreased FT3 and equivalent FT4 levels. CONCLUSIONS: The FT3/FT4 ratio appears to be constant between the ages of 3–17 y. Children on L-T4 due to congenital thyroid a/hypoplasia or central hypothyroidism have a decreased FT3/FT4 ratio compared to short normal children.