Three receptors, three different sensitivities
GLP-3R is not a single-target compound. It activates three receptors at once: GLP-1R, GIPR, and GCGR. Each of those receptors responds to the compound differently, and that difference is the whole reason the 4mg question keeps coming up.
The GIP receptor is the most sensitive. GLP-1 sits in the middle. And the glucagon receptor is the least sensitive of the three. Think of them like three volume knobs on a mixing board where GIP is already near full volume at low amounts, GLP-1 is mid-range, and glucagon needs a higher input signal before it starts contributing meaningfully to the mix.
So as the weekly amount increases across study participants, glucagon receptor activity follows behind the other two. It was never absent, it just needed more to get going.
What EC50 actually tells you
The way researchers measure receptor sensitivity is with a value called EC50. It is really just asking one thing: how much of this compound do you need before the receptor reaches half its maximum response?
A low EC50 means the receptor is sensitive. A high EC50 means you need more to get the same level of activation. For GLP-3R, the GIP receptor has the lowest EC50 of the three, meaning it responds at relatively low amounts. The glucagon receptor has the highest EC50, meaning it takes more before it is meaningfully engaged.
Potency comparison from preclinical data
Receptor activation is not a light switch
Receptor activation does not snap on at a specific amount. The activation follows a sigmoidal dose-response curve, which sounds complicated but really just means S-shaped.
At low amounts, the response is small but real. Through the middle of the curve, small increases in amount produce large jumps in effect. Near the top, the curve flattens out and adding more stops making much difference. The glucagon receptor's response to GLP-3R follows this exact pattern. At lower weekly amounts, there is some activation. It just is not large enough to show up as a clear signal in clinical measurements.
See it for yourself
Where the 4mg reference actually comes from
The 4mg figure does not come from receptor pharmacology directly. It comes from what researchers could actually measure in real people.
Beta-hydroxybutyrate and body composition data
In the Phase 2 liver fat sub-study, researchers tracked a biomarker called beta-hydroxybutyrate (BHB). BHB is produced by the liver when it is actively burning fat for fuel, and it is a reliable signal that the glucagon receptor is doing work. At weekly amounts of 4mg and above, BHB levels increased 2-3x above baseline. Below that, the increase was not statistically meaningful in that study's sample size.
That BHB jump at 4mg also lined up with the most significant changes in body composition measured by DEXA scans. Participants at 4mg and above showed the largest drops in total fat mass, with the 4mg group achieving around 15% reduction in total fat from baseline by week 36. The changes were real, measurable, and consistent with glucagon-driven fat oxidation becoming the dominant mechanism at that level.
At lower weekly amounts, the fat loss was still meaningful, mostly driven by GLP-1-driven caloric restriction. The glucagon signal simply was not large enough to register as a separate measurable contribution on top of that.
Figure 1. Percent change in body weight over 48 weeks across GLP-3R concentration groups vs. placebo. The separation between the 4mg group and lower amounts is visible from roughly week 16 onward, consistent with glucagon-driven fat oxidation becoming a measurable contributor at that level.
Source: Jastreboff et al., New England Journal of Medicine, 2023. Figure reproduced for educational reference only.
A useful simplification vs. a hard fact
Saying "the glucagon kicks in at 4mg" is a simplification. It is also a genuinely useful one.
When someone needs a practical reference point, "4mg is where the glucagon-driven effects became clearly measurable" is much more actionable than "it depends on your individual sigmoidal dose-response curve parameters." One of those answers helps. The other one just sounds impressive.
The problem only starts when the simplification gets treated as a hard threshold. The glucagon receptor was not off below 4mg. It was contributing, just not enough to separate its signal from the background noise of GLP-1 activity in a study of that size. Measuring tools have limits. That is a measurement constraint, not a biology fact.
What this means practically
The study behind the data
The liver fat sub-study data referenced here comes from the Phase 2a trial published in Nature Medicine in 2024. The primary obesity trial was published separately in the New England Journal of Medicine in 2023 and included the DEXA body composition sub-study.
The liver fat trial enrolled 98 participants with confirmed MASLD and at least 10% baseline liver fat content. After 24 weeks, liver fat reductions were 43% at 1mg, 57% at 4mg, 81% at 8mg, and 82% at 12mg versus no meaningful change in the placebo group. That jump from 57% to 81% between 4mg and 8mg is what made researchers pay attention. Combined with the BHB signal appearing clearly at 4mg, that is what anchored the reference point in the first place.
Primary source
Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial
Supporting source
Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial