IL-6: the signal upstream of CRP

Interleukin-6 (IL-6) is a cytokine, a signalling protein, released by immune cells, fat tissue and working muscle. In the liver it switches on production of C-reactive protein, fibrinogen and other acute-phase proteins, which is why those markers tend to move together. It is reported in picograms per millilitre (pg/mL), a concentration roughly a million times lower than CRP.

The ranges

IL-6 is not part of any cardiovascular or metabolic guideline. The reference value above describes where healthy people fall on one assay, and the lab notes that a normal result does not rule out inflammation. Values seen online described as "optimal IL-6" are not based on any guideline. In research, IL-6 is analysed as a continuous measure, usually on a log scale.

What raises and lowers it

• Exercise: working muscle releases IL-6, and levels can rise many-fold during long or hard exercise before falling back within hours. This is normal physiology, not disease.
• Infection, injury, surgery and inflammatory disease raise it sharply.
• Chronic, modest rises: excess visceral fat, smoking, poor sleep, ageing, and chronic kidney disease.
• Lowers: weight loss and fitness over time. Drugs that block the IL-6 receptor, such as tocilizumab for rheumatoid arthritis, lower CRP but can raise measured IL-6, because the cytokine has nowhere to bind.

What it predicts

A 2014 meta-analysis in the European Heart Journal combined a new case-cohort study with up to 29 prospective studies. Per one standard deviation higher log IL-6, the adjusted relative risk of non-fatal heart attack or coronary death was 1.25 (95% CI 1.19 to 1.32), the strongest of the cytokines studied. TNF-alpha was 1.17 and IL-18 1.13. These are associations, and the authors said more work was needed to establish cause.

Genetics strengthens the case. A 2012 Mendelian randomisation study in the Lancet, with up to 133,449 people, looked at a common variant in the IL-6 receptor gene that dampens IL-6 signalling. Carriers had lower CRP and fibrinogen and a lower risk of coronary heart disease, an odds ratio of 0.95 per copy. That points to IL-6 signalling as a cause of heart disease, not just a marker, and it is the rationale for outcome trials of IL-6-blocking drugs in cardiology. Those trials have to be read when they report; this page does not assume their result.

On GLP-1 drugs and weight loss

The pivotal semaglutide and tirzepatide trials do not report IL-6. Their effect on CRP is well documented (a 44% placebo-adjusted fall in STEP 1), and since IL-6 drives CRP a fall in both is plausible. The closest published data are for retatrutide, which is investigational: a 2026 post hoc analysis of a phase 2 trial in adults with obesity without diabetes reported a 29.6% placebo-adjusted fall in IL-6 over 48 weeks. In the companion trial in people with type 2 diabetes, IL-6 and hs-CRP did not fall significantly.

How to test

• No exercise on the day of the draw, and no hard or long session for 48 hours beforehand.
• Be well. No recent infection, vaccination, injury or dental work.
• Same time of day. IL-6 has a daily rhythm and is affected by poor sleep the night before.
• Same lab. IL-6 assays are not standardised against each other, and sample handling affects the result.
• Read it with hs-CRP from the same draw. If IL-6 is up and CRP is not, look for an exercise or handling explanation first.

Tracking it in Peptide AI

In Peptide AI, IL-6 sits in the inflammatory panel. The app reads it from a lab PDF or manual entry, scores the panel from 0 to 100 alongside an overall grade, flags values that warrant prompt physician consultation, and plots each result against your dose timeline. Training logged in the training log sits on the same timeline, which is the first thing to check against a high value. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.