We studied growth hormone (GH) stimulation and insulin-like growth factor -I (IGF-I) generation tests in 15 children with neglected congenital hypothyroidism (CH) (age = 6.4 +/- 4.2 years) and measured their growth parameters for >1 years after starting thyroxine (T4) replacement. One year after treatment, height SDS (HtSDS) increased from -4.3 +/- 2.5 to -2.7 +/- 2.3. Peak GH response to clonidine increased from 3.2 +/- 1.2 ng ml(-1) to 7.62 +/- 1.38 ng ml(-1) after treatments. Basal and peak IGF-I response to GH increased from (34.66 +/- 17.3 ng ml(-1) and 58.4 +/- 36.99 ng ml(-1), respectively) before treatment to (130.6 +/- 97.8 ng ml(-1) and 193.75 +/- 122.5 ng ml(-1), respectively). HtSDS increments were correlated significantly with basal free T4 concentrations (r = 0.622, P < 0.01). In summary, after long period of hypothyroidism, T4 replacement produced significant, although incomplete, catch-up growth through a partial recovery of GH- IGF-I axis.