Effects of levothyroxine on growth hormone (gh) sensitivity in children with idiopathic short stature.

BACKGROUND: The possible relationship between the circulating concentrations of T4 and GH sensitivity has not been elucidated. OBJECTIVE: The aim of this study is to evaluate the effect of levothyroxine supplementation on GH sensitivity in prepubertal boys with idiopathic short stature (ISS). METHODS: We selected 28 prepubertal boys with ISS (mean age 8.2±0.5years) and free T4 (Ft4) concentrations between the 3rd and the 25th percentiles (Ft4: 0.8-1.5ng/dl). They were randomly divided into two groups: Group A received thyroid supplementation (1-3μg/kg/day) for 120days, and Group B received placebo for the same period. To evaluate GH sensitivity, an IGF-I generation test (GH: 33μg/kg/day sc for 3days) was performed in both groups: under basal conditions, and after 120days of levothyroxine supplementation (or placebo). RESULTS: After thyroid supplementation, Group A had higher Ft4 concentrations compared with Group B (2.14±0.06 vs 1.48±0.06ng/dl, p=0.01), their growth velocity was significantly higher (2.3±0.1 vs 1.5±0.2cm/4months), and they exhibited a greater increase in IGF-I after GH administration (Group A: 32.5±3.8% vs Group B 17.3±2.6%). CONCLUSION: Supplementation with levothyroxine for 120days promotes an increase in growth velocity, and a greater IGF-I response to short-term GH administration in prepubertal boys with ISS and low-normal thyroid hormone concentrations.