Effect of growth hormone on growth and myelination in the neonatal hypothyroid rat.

The possible involvement of a deficit of GH and insulin-like growth factor-I (somatomedin C) (IGF-I/SMC) in mediating the effects of propylthiouracil (PTU)-induced hypothyroidism on body and skeletal growth and myelination was studied in the neonatal rat. Myelination (as assessed by 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) activity), skeletal growth (as assessed by tail length) and body weight of pups from PTU-treated mothers were significantly retarded compared with normal animals or euthyroid controls. At 20 days after birth, plasma GH in hypothyroid animals was undetectable (less than 10 micrograms/l), pituitary GH content was 1.2% of control, and plasma, liver and kidney IGF-I/SMC concentrations were 63, 68 and 50% of control values respectively. CNP activity in hypothyroid brain was 52% of normal controls but the concentration of IGF-I/SMC was 113-154% of control. Treatment of hypothyroid animals from day 1 with GH (10 mg/kg body weight per day) restored liver and plasma IGF-I/SMC concentrations at 20 days to values above those of normal animals and euthyroid controls. The concentration of IGF-I/SMC was also significantly (P less than 0.001) restored in hypothyroid kidney (79% of normal), but the concentration in brain was unaffected. These observations provide evidence that the GH treatment employed in the present experiments was adequate to restore the deficit. GH treatment had no significant effect on tail length or CNP activity, and only a small (4-24%) effect on body weight at 20 days. Only thyroxine was able fully to restore body weight and substantially restore tail length and CNP activity.(ABSTRACT TRUNCATED AT 250 WORDS)