The catabolic state of prolonged critical illness is associated with a low activity of the thyrotropic and the somatotropic axes. The neuroendocrine component in the pathogenesis of these low activity states was assessed by investigating the effects of continuous intravenous infusions of TRH, GH-releasing peptide-2 (GHRP-2), and GHRH. Twenty adult patients, critically ill for several weeks, were studied during two consecutive nights. They had been randomly allocated to one of three combinations of peptide infusions, each administered in random order: TRH (one night) and placebo (other night), TRH + GHRP-2 (one night) and GHRP-2 (other night), or TRH + GHRH + GHRP-2 (one night) and GHRH + GHRP-2 (other night). The peptide infusions were started after a 1-μg/kg bolus and infused (1 μg/kg per h) until 0600 h. Blood sampling was performed every 20 min, and pituitary hormone secretion was quantified by deconvolution analysis. Reduced pulsatile fraction of TSH, GH, and PRL secretion and low serum concentrations of T₄, T₃, in-sulin growth factor-I (IGF-I), IGF-binding protein-3 (IGFBP-3), and the acid-labile subunit (ALS) were documented in the untreated state.

Infusion of TRH alone or in combination with GH secretagogues augmented nonpulsatile TSH release 2- to 5-fold; only TRH + GHRP-2 increased pulsatile TSH secretion (4-fold). Average rises in T₄ (40–54%) and in T₃ (52–116%) were obtained with all three combinations, whereas reverse T₃ levels did not increase, except when TRH was infused alone. Pulsatile GH secretion was amplified >6- and >10-fold, respectively, by GHRP-2 and GHRH + GHRP-2 infusions, generating mean increases of serum IGF-I (66% and 106%), IGFBP-3 (50% and 56%), and ALS (65% and 97%) within 45 h. The addition of TRH did not alter the GH secretory patterns. TRH infusion increased PRL release only when combined with GH secretagogues. No effects on serum cortisol were detected.

In conclusion, the pathogenesis of the low activity state of the thyrotropic and somatotropic axes in prolonged critical illness appears to have a neuroendocrine component, because these axes are both readily activated by coinfusion of TRH and GH secretagogues.