Renal handling of phosphate can predict height velocity during growth hormone therapy for short children.

Renal tubular reabsorption of phosphate in response to GH administration was studied in 28 short Japanese children, aged 5-11 yr (height SD score, less than -2.0 SD). Three groups included a classical GH deficiency (group 1; n = 12), a partial GH deficiency (group 2; n = 7), and children with non-GH deficiency (group 3; n = 9), depending on the peak response of serum GH in four provocative tests. Serum phosphorus, alkaline phosphatase, insulin-like growth factor-I (IGF-I), osteocalcin, and ratio of the maximum tubular reabsorption rate for phosphorus to the glomerular filtration rate (Tmp/GFR) were all significantly lower in group 1 compared with findings in groups 2 and 3 (P less than 0.05, P less than 0.01, and P less than 0.001). After the administration of GH (0.1 U/kg.day) for 4 consecutive days, increments in serum phosphorus and Tmp/GFR were significantly higher in group 1 than in group 2 (P less than 0.01 and P less than 0.01) or group 3 (P less than 0.01 and P less than 0.01), whereas the increment in IGF-I was similar in all 3 groups, and the levels of serum alkaline phosphatase and osteocalcin remained unchanged in all 3 groups. The calculated ratio of the increment in Tmp/GFR to the increment in IGF-I (delta Tmp/GFR/delta IGF-I) was highest in group 1, intermediate in group 2, and lowest in group 3 (P less than 0.001). One year after the GH treatment (0.5 U/kg.week), height velocity was 7.9 +/- 2.2 cm/yr in group 1 and 5.9 +/- 1.2 cm/yr in group 2; no child in group 3 was treated. When the above calculated parameters, delta Tmp/GFR/delta IGF-I and increment in height velocity (difference between pre- and posttherapy values), were taken into account, there was a significant positive correlation (n = 19; r = 0.78; P less than 0.001). This parameter can be used for purposes of predicting the outcome after 1 yr of GH therapy.