The effect of short-term hypothyroidism on growth hormone secretory responses to growth hormone-releasing hormone (1-29) and insulin-induced hypoglycemia

ABSTRACT

Introduction: Hypothyroidism is frequently associated with growth failure. In long-standing hypothyroidism the growth hormone (GH) secretory responses to growth hormone-releasing hormone (GRH) and insulin-induced hypoglycemia are decreased. Methods: To investigate whether hypothyroidism of short duration affects pituitary GH release, we measured the serum GH response to synthetic GRH (1-29), 1g/kg body weight, given IV to six athyreotic patients during thyroxine (T4) treatment and one month after stopping T4 (short-term hypothyroid state). This served as a direct measure of pituitary somatotroph function .We also assessed the serum GH response to insulin-induced hypoglycemia in three patients as an indirect assessment of hypothalamic function. Results: We found that basal GH levels remained the same both during (euthyroid state) and after stopping T4 therapy (hypothyroid state). Peak serum GH response to GRH was significantly greater in patients while they were hypothyroid than during T4 therapy when they were euthyroid (p< 0.01). There was no difference in the peak serum GH response to hypoglycemia during and after stopping T4 replacement (30.3 + 8.9 g/L euthyroid-state versus 45 + 14.5 g/L short term hypothyroid-state). Discussion: These results suggest that, in contrast to long standing hypothyroidism, thyroid hormone deficiency of short duration increases somatotroph sensitivity to GRH, perhaps as a result of decreased endogenous hypothalamic IGF-1 release and/or tone.