Building muscle looks simple from the outside. You train, you eat your protein, and the muscle grows. But training is only the trigger. Whether your body actually answers that trigger is decided by what is happening in your blood, and two things have to be in place at once. There has to be a signal telling the muscle to grow, and there has to be enough fuel to do the repair work. When either one is missing, the muscle stalls, and that stall almost never announces itself in the mirror. It shows up on a blood panel.

This article walks through the five markers that most often block muscle in people who are already doing the work, why a result can read normal and still be the thing holding you back, and the one intake problem that a GLP-1 (a class of appetite-lowering compounds) quietly creates. It finishes with where a compound can actually help and, just as important, the markers no compound in this space can touch. Everything here is framed for research, not as medical advice, and the honest first step for most of it is a blood test read with a doctor.

A hard set does not build muscle by itself. It sends a request. Your body either answers that request or it does not, and the answer depends on two separate things being in place. The first is a growth signal, the hormonal message that tells muscle tissue to build. The second is fuel, the oxygen and raw materials that let the repair actually happen. Miss the signal and you tear the muscle down in training without building it back. Miss the fuel and you run out of capacity before the work is done. Either way the result is the same, a stall that looks like bad genetics or a bad program when it is really a number on a panel.

Reference ranges are built from population averages. Normal simply means where most people land, which includes plenty of people who feel flat and are not building anything. That is why a marker can read normal on paper and still be the reason you are stuck. This is not a reason to chase arbitrary numbers or to self-treat toward a target, and where a result sits inside or outside a range is a conversation for a doctor. The point is narrower than that. If you are doing the work and nothing is happening, a result that looks fine at a glance is still worth a closer look, because normal and optimal are not the same thing.

Testosterone is the main hormone that tells the body to build muscle, but most of the testosterone in your blood is not actually available. A large share of it is bound to a protein and cannot act on tissue. Only the fraction that circulates unbound, called free testosterone, is available to do the building. Most doctors measure total testosterone by default, which counts the bound and the free together. That is the first trap. A total number can look perfectly healthy while the free fraction underneath it is too low to drive growth.

The protein doing most of that binding is SHBG, short for sex hormone binding globulin, a carrier protein in the blood. The simplest way to picture it is a sponge. SHBG soaks up testosterone and holds onto it, and anything stuck to that sponge cannot build. The more SHBG you carry, the less testosterone stays free, so when SHBG runs high your free testosterone falls even when your total looks fine. This is the actual reason two men with identical total testosterone can end up in different places. One has low SHBG and plenty of free testosterone to work with. The other has high SHBG and very little left over. A long aggressive diet can push SHBG up on its own, and so can getting older, which is one of the quiet reasons the build signal goes missing with age. Low free testosterone tends to feel like flat workouts, strength that will not climb, and recovery that never fully catches up, though symptoms like these are nonspecific and only a panel can confirm what is actually going on.

If testosterone tells the muscle to grow, IGF-1 is the messenger that carries out much of the repair. IGF-1 stands for insulin like growth factor one, and it is the signal behind recovery and the building of new tissue. Your body makes most of its IGF-1 in the liver in response to growth hormone, which is released in pulses from the pituitary, a small gland at the base of the brain. When IGF-1 runs low, recovery slows down. You still break the muscle down during training, but the repair never fully lands, so the work tears you down without building you back up. Research suggests that low IGF-1 blunts growth even when training is hard and consistent. In practice it tends to feel like soreness that lingers for days and progress that stalls no matter how disciplined you are.

The last two markers are not about a signal at all. They are about fuel. The first is ferritin, which is stored iron. Iron is what carries oxygen in the blood, and oxygen is what your muscles run on when you train. When ferritin is low, oxygen delivery drops, so you gas out early, your lifts stall, and recovery between sessions drags. Low ferritin is common in people who train hard, and it is more common still when red meat intake is low, which often happens on its own when appetite falls on a GLP-1.

The second fuel marker is B12, a vitamin your body needs to make red blood cells, which are what carry that oxygen around in the first place. B12 also helps keep energy steady, and when it runs low you feel flat and worn out and your training quality drops with it. B12 comes almost entirely from animal foods like meat and eggs, so when appetite drops on a GLP-1, B12 tends to drop too. Low ferritin and low B12 often show up together, and both drain the same thing, the energy that training and recovery depend on.

Three cover the growth signal and two cover the fuel. Each one can stall muscle on its own, and none of them is visible from the outside.

Protein is the raw material muscle is built from. Resistance training sends the signal, the markers above decide whether the body can answer, and protein is the material the answer is built out of. This is where GLP-1 compounds like retatrutide quietly work against you. A GLP-1 lowers appetite across the board, which is the point when the goal is fat loss, but protein is usually the first thing to fall short when total intake drops. You end up in a deficit with too little material to rebuild from, and that alone can cost you muscle before any marker is even measured. The takeaway is not that a GLP-1 is the enemy. It is that on a GLP-1, protein intake and resistance training matter more, not less, because the appetite suppression is actively pulling the raw material away. Protein and training set the ceiling for muscle. Your blood decides how much of that ceiling you can reach.

Once you can see the markers, the question becomes where a compound actually helps. The honest answer is that it depends entirely on which marker is the problem, and most of the levers people reach for touch only one part of the picture.

Start with high SHBG. Research associates higher circulating testosterone with lower SHBG, because more testosterone in circulation tends to signal the liver to produce less of the binding protein. Whether that route makes sense for you is a decision for a doctor working from your actual panel, not something to guess at, because it is a hormonal change with its own tradeoffs.

Low IGF-1 is where growth hormone compounds come in. A GH secretagogue like tesamorelin or CJC-1295 can support the growth signal. Secretagogue just means it tells a gland to release something, and in this case it asks your pituitary to release more of its own growth hormone. The catch is built into the mechanism. It works only through your own pituitary, so if that gland is already running low there is little left to amplify, and a low IGF-1 on your panel can be exactly the sign that the reserve is limited. A secretagogue can help when the pituitary still has room to respond, but it cannot raise a low free testosterone, it cannot bring down a high SHBG, and it cannot refill low iron or low B12.

This is where exogenous HGH, meaning growth hormone taken from outside the body, enters the research conversation. HGH is different from a secretagogue in one key way. It does not ask the pituitary for anything. It delivers growth hormone directly and is not capped by how well your own gland responds, and research suggests the IGF-1 response from exogenous HGH is often stronger as a result. That is a more direct route to the same lean mass signal, but it carries the exact same blind spot as a secretagogue. It does not raise low free testosterone, it does not lower high SHBG, and it does not refill iron or B12 either. On top of that it comes with a real tradeoff. A steady supply from outside can turn down your own production over time, because the body senses growth hormone is already high and makes less of its own, which is not true of a secretagogue working inside your own system. Further downstream again is IGF-1 LR3, a modified form of the IGF-1 signal itself that bypasses growth hormone entirely, which is a different decision with its own considerations and is covered separately in our breakdown of HGH versus IGF-1 LR3 for muscle.

The pattern underneath all of this is the part worth keeping. There are five markers that commonly block muscle, and no growth hormone compound touches four of them. A secretagogue or HGH can support the IGF-1 signal under the right conditions, but free testosterone, SHBG, ferritin, and B12 all sit completely outside what they can reach. That is why the blood work comes first.

For research and educational purposes only. Not medical advice.