Growth hormone responses to growth hormone-releasing factor (1–29) in euthyroid, hypothyroid and hyperthyroid rats

Abstract

ABSTRACT In order to investigate whether the impaired GH secretion associated with hypothyroidism and hyperthyroidism is due to a hypothalamic or a pituitary disorder, we have studied plasma GH responses to GH-releasing factor (1–29) (GRF) in euthyroid, hypothyroid and hyperthyroid rats. Hypothyroid rats showed a significant (P< 0·001) reduction in GH responses to GRF (5 μg/kg) at 5 min (350 ± 35 vs 1950 ±260 μg/l), 10 min (366±66 vs 2320 ± 270 μg/l) and 15 min after GRF injection (395 ± 72 vs 1420 ± 183 μg/l; means ± s.e.m.) compared with euthyroid rats. Hyperthyroid rats showed a significant (P<0·05) decrease in GH responses to 5 μg GRF/kg after 30 min (200±14 vs 325 ± 35 μg/l) but not at other time-points, or after the administration of 1 μg GRF/kg. These data indicate that in hypothyroidism and perhaps hyperthyroidism there is an alteration in the responsiveness of the somatotroph to GRF administration. J. Endocr (1986) 109, 53–56