Catch-up growth after childhood-onset substitution in primary hypothyroidism: Is it a guide towards optimal growth hormone treatment in idiopathic growth hormone deficiency?

Catch-up growth was analyzed in 20 prepubertal children with primary hypoth yroidism (PH) starting treatment at an age of 4.4 (1.2-10.1) years and a he ight (HT) SD score (HT SDS) of -3.1 (+/- 0.8). All patients were followed f or at least 3 prepubertal years. HT velocity was 12.3 +/- 2.3, 9.0 +/- 1.8 and 7.5 +/- 2.2 cm/year, and change in HT SDS was 1.60 +/- 0.56, 0.57 +/- 0 .33 and 0.28 +/- 0.38 during the 1st, 2nd and 3rd year, respectively. The 1 1 children followed to adult height reached a HT SDS of -0.11 +/- 1.1, all within their target HT range. HT gain (Delta HT SDS) during the ist year wa s correlated with the degree of catch-up growth (r(2) = 0.78, p < 0.001). W hile catch-up growth in childhood-onset PH is complete, this is not the cas e in GH deficiency (GHD). Based on the auxological characteristics of the p atients with PH, HT velocities during the first 2 years were predicted appl ying prediction models devised for prepubertal children with idiopathic GHD . The modalities of GH treatment observed in the models were used to calcul ate predicted HT velocities of the PH patients. Observed HT velocities in P H were higher than predicted HT velocities during the 1st (10.67 +/- 1.37 c m/year, p < 0.01) and 2nd (8.35 +/- 0.86 cm/year, p = 0.128) year. The data show that catch-up potential in idiopathic GHD of childhood onset is reduc ed compared to PH. Since early catch-up as well as total HT recovery in chi ldren with GHD are often not reached by present treatment modalities, catch -up growth in PH may serve as a model towards optimizing GH treatment. The data suggest that initial GH doses of 1.0 IU/kg/week, rather than the prese ntly recommended 0.6 IU/kg/week, need to be given in GHD in order to achiev e the degree of early catch-up observed in PH and to consequently improve t he final outcome.