You can nail your training plan, hit your protein, and get to bed on time, and still feel stuck. Sometimes what's holding you back isn't happening in the gym at all. It's happening in your blood.
Sleep, food, and stress all shape how you perform and recover. So do a handful of biomarkers most people never look at: iron, cortisol, key vitamins, inflammation, thyroid, and hormones. When one of them is off, consistent training can only take you so far.
Here are the biomarkers worth knowing, what each one does, and how it can show up in your workouts.
Hemoglobin & Ferritin
If you've ever felt winded on a run that should have been easy, oxygen delivery is a good place to start looking. Hemoglobin and ferritin are two of the biggest players.
Here's why they matter:
Hemoglobin gets oxygen to your muscles.
It's the protein in your red blood cells that picks up oxygen in your lungs and drops it off at your working muscles. That oxygen powers your endurance, your strength, and your recovery.
When hemoglobin runs low, you feel it.
Your muscles get less oxygen, and that can show up as fatigue, shortness of breath, dizziness, or running out of gas earlier than you should.
It affects strength and recovery, too.
Without enough oxygen, your muscles can't work as hard or bounce back as quickly. Training starts to feel harder than it should, even when your nutrition and programming are dialed in.
Ferritin is your iron savings account.
Ferritin stores iron so your body has a steady supply when it needs it. Your iron can look normal on paper while your ferritin is low, and your performance can still take a hit.
Low ferritin means less in reserve.
When your stores run low, your body has a harder time keeping up with the demands of training. Endurance and recovery are usually the first to suffer.
Iron rounds out the picture.
Iron in your blood shows what's available today. Ferritin shows what's in the tank. Looking at both alongside hemoglobin helps show whether low energy is coming from your iron levels or from something else.
If you're unusually tired or out of breath during workouts, check your hemoglobin and ferritin. Getting both where they should be can make a real difference in how you feel and perform.
Cortisol
Cortisol gets blamed for a lot, but you actually need it to train well. It's your body's main stress hormone, and the goal isn't to keep it as low as possible. You want cortisol that shows up when you need it and backs off when you don't.
Here's what to know:
It helps fuel your workouts.
Cortisol rises during exercise and helps your body pull energy from stored carbs, fat, and protein, so your muscles have something to work with.
Then it should settle back down.
After you train, cortisol normally drops back toward your baseline. That rise and fall is what a healthy stress response looks like.
Staying high for too long can hold you back.
Hard training without enough rest, short nights, or a rough stretch at work can keep your stress response running hot. Over time, that makes it harder to recover and build muscle.
Too little isn't great either.
Low cortisol can leave you drained and make workouts feel harder than they should.
Timing changes the number.
Cortisol peaks shortly after you wake up and drops through the day, so the time of your blood draw affects your result.
A single cortisol result can't tell you whether you're overtrained. Paired with how you're sleeping, recovering, and feeling, though, it adds useful context. That's where talking it through with your physician helps.
Vitamin B12 & Vitamin D
These two vitamins do a lot of quiet work behind the scenes. You might not notice them when they're in a good range, but you'll notice when they're not.
Here's why they matter:
B12 helps turn food into energy.
It also helps your body make red blood cells, which carry oxygen to your muscles while you train.
Low B12 can drain your energy.
A deficiency can cause fatigue, weakness, and even numbness or tingling. That makes it hard to train consistently and recover well.
Vitamin D supports your muscles and immune system.
It also helps regulate calcium, which your muscles need to contract properly.
Low vitamin D is linked to lower strength and higher injury risk.
It can affect your mood too, which can quietly chip away at your motivation and consistency.
Keep B12 and D in a healthy range, and you're giving your energy, muscles, and recovery a solid foundation.
High-Sensitivity C-Reactive Protein (hs-CRP)
hs-CRP measures inflammation. The "high-sensitivity" part means it can catch low levels that a standard CRP test might miss, which makes it good at spotting inflammation that's quietly hanging around.
Here's what to know:
Regular exercise tends to bring it down.
Over time, consistent training is linked to lower hs-CRP levels.
A tough workout can bump it up for a bit.
After a long or intense session, hs-CRP can climb, often peaking the next day and settling back down within a few days. That's normal.
Levels that stay high deserve a closer look.
If your hs-CRP is elevated even when you're well rested, it could point to ongoing inflammation from under-recovery, poor sleep, illness, or other health factors.
It's not just about your workouts.
Low-grade inflammation is also tied to heart and metabolic health. Keeping it in check helps you train well now and stay healthy for the long haul.
Tracking hs-CRP shows you whether your body is keeping up with your training or asking for more recovery.
Thyroid-Stimulating Hormone (TSH)
Your thyroid sets the pace for a lot of what your body does, including how you use energy and how your muscles and heart work. TSH is the signal your brain sends to tell your thyroid how hard to work, which is why it's usually the first thyroid marker doctors check.
Here's why it matters:
Thyroid hormones help power your muscles.
They help your muscle cells turn fuel into energy you can actually use.
An underactive thyroid can make every workout feel harder.
Low thyroid function is linked to fatigue, muscle weakness, cramps, and a lower tolerance for exercise.
So can an overactive one.
Too much thyroid hormone can also cause muscle weakness and make exercise harder to handle.
TSH is where you start.
A high TSH often means your thyroid is underactive. A low TSH can mean it's overactive. Free T4 and Free T3 fill in the rest of the story.
If you're sleeping well, eating well, and training smart but still feel like you're running on empty, your thyroid is worth a look.
Testosterone & Estradiol
People tend to file testosterone under "male" and estradiol under "female," but everyone needs both. Together, they shape how strong you get, how well you recover, and how your body holds up over time.
Here's what to know:
Testosterone helps you build and keep muscle.
Healthy levels support muscle growth, strength, and power.
It helps you recover.
Testosterone plays a role in protein synthesis, the process your muscles use to repair and grow after a workout.
When testosterone drops, strength and energy often follow.
It can also lead to slower recovery, more fat gain, and less motivation to train.
Estradiol matters for everyone.
It's the main form of estrogen, and it supports bone, heart, and metabolic health in both men and women.
It helps keep your bones strong.
That matters more as your training volume climbs and as you get older.
Under-fueling can drag it down.
When you don't eat enough to cover your training, your body may scale back hormone production. In women, that can show up as irregular or missed periods. Low estrogen combined with low energy can weaken bones and raise the risk of stress fractures.
Balance is the goal.
Your panel also measures SHBG, a protein that affects how much testosterone and estradiol your body can actually use.
Whatever your sex, balanced testosterone and estradiol help you build strength, recover well, and protect your muscles and bones as you age.
The Bottom Line
How hard you train is only part of the story. What's happening inside your body matters just as much. Checking hemoglobin, ferritin, cortisol, vitamins B12 and D, hs-CRP, thyroid, testosterone, and estradiol shows you how your body is handling your training, where it might be struggling, and what to change so you can perform and recover better.
Thrive's Healthspan tests every biomarker in this post as part of a panel of 90+ biomarkers. You'll review your results one-on-one with a physician and leave with a personalized plan built around your results and goals.
This article is for informational purposes only and does not constitute medical advice. The information contained herein is not a substitute for and should never be relied upon for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.
