Two ways to ask for growth

The pituitary releases growth hormone in bursts. Several times a day, most of them at night, the gland fires a pulse; between pulses the level drops close to nothing. That pattern is not an accident of plumbing. It is the signal, and the whole axis is built to read a rhythm rather than a concentration — the same principle that governs the reproductive axis, running one gland over.

Two hypothalamic signals set that rhythm, and they pull in opposite directions. GHRH — growth-hormone-releasing hormone — is the accelerator; somatostatin is the brake. A GH pulse is what you get when GHRH rises while somatostatin falls. And there is a second accelerator the body discovered later: ghrelin, the stomach’s hunger hormone, which turns out to drive GH through a receptor all its own. So there are exactly two buttons you can press to ask for more growth hormone without injecting the hormone itself — push GHRH, or press ghrelin’s receptor — and the entire secretagogue shelf is one or the other, or both at once.

The obvious alternative is to supply GH directly, and for genuine deficiency that is the treatment. But exogenous GH flattens the rhythm. A daily injection holds a broad, unphysiological plateau where the body expects sharp peaks over empty troughs, and it switches off the gland’s own output through the feedback loop that normally governs it. The pattern the tissues are tuned to read is simply gone.

A secretagogue works the other way. It raises the gland’s own secretion, so whatever pulse structure the pituitary can still produce is preserved, and the downstream brake stays connected: IGF-1, made in the liver in response to GH, feeds back to restrain both GHRH and the pituitary. That feedback is why a secretagogue cannot drive GH without limit — the higher IGF-1 climbs, the harder the axis pushes back. It is the real mechanistic advantage of the approach, and it is also a ceiling: you are asking a gland that may already be near its output, not bypassing it.

Native GHRH is a 44-residue peptide, and its business end is the first 29 residues — GRF(1–29), the fragment that retains full activity. Nearly every GHRH drug is that fragment, re-engineered for survival in the blood. Sermorelin is GRF(1–29) essentially unmodified; it was once an approved product (Geref) for diagnosing and treating childhood GH deficiency and was withdrawn from the US market for commercial, not safety, reasons. It works, but it clears in minutes.

Tesamorelin is the same fragment with a stabilizing group on the N-terminus, and it is the one GHRH analog that carries a real outcome trial and an FDA label. In a randomized study of 412 people with HIV-associated abdominal fat accumulation, daily tesamorelin cut visceral adipose tissue by about 15% while placebo rose (Falutz et al., NEJM, 2007). That is the high-water mark for evidence in this entire family — one molecule, one population, one body-composition endpoint.

CJC-1295 is where the engineering gets interesting, and where the pulsatility argument starts to wobble. It comes in two forms. The short-acting fragment (often sold as “mod GRF 1-29”) clears fast; a single dose was shown to raise trough and 24-hour GH while leaving pulse frequency and amplitude intact (Ionescu & Frohman, JCEM, 2006) — pulsatility genuinely preserved. The long-acting “CJC-1295 with DAC” form adds a group that binds albumin and keeps the molecule circulating for days. That is marketed as an advantage, and in half-life terms it is. But a GHRH signal that never switches off is, by definition, the continuous plateau the whole approach was supposed to avoid. Stretch the half-life far enough and the accelerator gets stuck down.

The other accelerator came from an unexpected place. In the 1980s, synthetic peptides were found to release GH through a receptor that was not the GHRH receptor — the growth-hormone-releasing peptides, or GHRPs. For years the natural ligand of that receptor was a mystery; when it was finally found, in 1999, it was ghrelin, a stomach hormone better known for driving hunger. The GHRPs had been mimicking a gut hormone all along, and the receptor is GHS-R1a, the ghrelin receptor.

The early GHRPs — GHRP-6, GHRP-2, hexarelin — were effective but blunt: they also bumped cortisol and prolactin, because the ghrelin receptor sits near circuits that control them. Ipamorelin was the fix. Described in 1998 as “the first selective growth hormone secretagogue,” it releases GH with a potency close to GHRP-6 but does not raise ACTH or cortisol even at doses far above the GH-release threshold (Raun et al., Eur J Endocrinol, 1998). Clean selectivity is why ipamorelin became the GHRP the community reaches for.

And this receptor, like the GLP-1 receptor, does not insist on being addressed by a peptide. MK-677 (ibutamoren) is an orally active, non-peptide ghrelin mimetic — a daily pill that hits GHS-R1a. It is the same small-molecule move the incretin field is making with oral GLP-1: the receptor reads the message, so the messenger can stop being an injection. It is also, as it happens, the secretagogue with the longest controlled human record — which is where the sceptical reading begins.

The reason the standard community stack pairs a GHRH analog with a GHRP — CJC-1295 with ipamorelin is the canonical example — is not marketing. It is one of the cleaner pharmacology results in the field. Because the two inputs land on different receptors, giving them together releases more GH than the sum of each alone. The synergy is not explained simply by the GHRP suppressing somatostatin; the peptides act at both hypothalamic and pituitary levels through a mechanism distinct from GHRH’s (Bowers et al., Endocrinology, 1991).

Read as signals, it makes sense: GHRH tells the somatotroph to release, a GHRP amplifies that release and lifts the somatostatin brake a little, and the pulse that results is larger than either lever could produce by itself. This is the genuinely elegant part of the story — two evolutionarily separate accelerators that happen to multiply. It is also the point where the biology is most settled and the human outcome data is thinnest.

Here the page turns sceptical. Everything above is real mechanism; the question is what it buys in a person. The honest pattern across the whole class is a split between surrogate markers, which move reliably, and outcomes, which mostly do not — or have not been tested. A secretagogue will raise your GH and IGF-1; that is easy to show and essentially guaranteed. Whether it makes you leaner, stronger, or better-recovered is a separate claim, and it is where the evidence runs out.

MK-677 is the clearest window, because it was studied for a year. In healthy older adults it restored GH and IGF-1 toward young-adult levels and added about a kilogram of fat-free mass — and strength and physical function did not improve, while fasting glucose rose (Nass et al., Ann Intern Med, 2008). The markers moved; the thing you actually wanted did not. Ipamorelin tells a similar story from the other direction: its one completed human efficacy trial, for post-operative ileus, failed. Tesamorelin keeps its label precisely because it cleared an outcome endpoint in a defined population — and even there the effect reverses when the drug stops.

The self-limiting feedback that makes secretagogues attractive is also a cap: IGF-1 feedback means you are coaxing a gland, not commanding it, so an older or already-maximal axis has little room to give. And none of these compounds is a free lunch pharmacologically — the ghrelin mimetics can raise appetite, blood sugar, and fluid retention, exactly as a hunger hormone acting through its own receptor would predict. Every one of them is banned in sport by WADA. Bullish on the biology, which is excellent; sceptical on the page, which keeps promising the outcome and delivering the marker.

• Two receptors, one gland. GHRH analogs (sermorelin, tesamorelin, CJC-1295) press the GHRH receptor; GHRPs and ghrelin mimetics (ipamorelin, MK-677) press GHS-R1a. Separate buttons, same pulse.
• Working with the rhythm is the point — and the catch. Secretagogues ask the gland to fire its own pulse and leave IGF-1 feedback intact. But long-acting CJC-1295-with-DAC flattens the very pulsatility that was the argument.
• Synergy is more than additive. Because the two inputs hit different receptors, GHRH + GHRP together release more GH than either alone — the basis of the CJC-1295 + ipamorelin stack, and the field's best-settled result.
• Selectivity was the real advance in GHRPs. Older GHRP-6 and GHRP-2 also raised cortisol and prolactin; ipamorelin releases GH without them, which is why it won out.
• Markers move; outcomes mostly don't. Tesamorelin has an approval and an outcome (visceral fat in HIV). MK-677 raised IGF-1 and lean mass for a year without improving strength, and nudged glucose up. Ipamorelin's one efficacy trial failed. All are WADA-banned.