Prolactin and Cortisol Cross-Stimulation

Cross-stimulation describes what happens when a compound intended to release growth hormone also releases other pituitary hormones. GHS-R1a is not confined to somatotrophs; it is expressed in hypothalamic nuclei and elsewhere in the pituitary, so agonists can drive corticotropin-releasing hormone and vasopressin pathways, raising ACTH and then cortisol, along with prolactin. GHRH-receptor agonists, working through a different receptor, generally do not do this.

The effect is not uniform across the class, and the differences are well characterised in short human studies. GHRP-6 and especially hexarelin produce measurable rises in ACTH, cortisol and prolactin; GHRP-2 produces a smaller effect; ipamorelin was selected during development precisely for releasing growth hormone without appreciable ACTH, cortisol or prolactin response. The magnitude is modest and transient after a single administration and attenuates with repeated exposure.

The distinction matters when a compound is being compared to another on receptor grounds alone. A cortisol response works against the metabolic effects usually being sought, and a sustained prolactin elevation has its own consequences, including suppression of gonadotropins with prolonged exposure. It is also why peptides in this class are unsuitable during adrenal axis assessment: they perturb the thing being measured.

The common misuse is generalising in either direction. Ipamorelin's selectivity is routinely quoted as though it were a property of secretagogues generally, and conversely hexarelin's cortisol data are applied to compounds that do not share the effect. Both errors come from treating the class as one molecule. A further overreach is calling measured cortisol and prolactin spikes inconsequential; single-dose data in healthy volunteers cannot establish that for sustained use.