Catch-up growth was analyzed in 20 prepubertal children with primary hypothyroidism (PH) starting treatment at an age of 4.4 (1.2–10.1) years and a height (HT) SD score (HT SDS) of –3.1 (±0.8). All patients were followed for 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 11 children followed to adult height reached a HT SDS of –0.11 ± 1.1, all within their target HT range. HT gain (ΔHT SDS) during the 1st year was correlated with the degree of catch-up growth (r<sup>2</sup> = 0.78, p < 0.001). While catch-up growth in childhood-onset PH is complete, this is not the case in GH deficiency (GHD). Based on the auxological characteristics of the patients with PH, HT velocities during the first 2 years were predicted applying prediction models devised for prepubertal children with idiopathic GHD. The modalities of GH treatment observed in the models were used to calculate predicted HT velocities of the PH patients. Observed HT velocities in PH were higher than predicted HT velocities during the 1st (10.67 ± 1.37 cm/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 reduced compared to PH. Since early catch-up as well as total HT recovery in children 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 presently recommended 0.6 IU/kg/week, need to be given in GHD in order to achieve the degree of early catch-up observed in PH and to consequently improve the final outcome.