Is IGF-1 LR3 the same as growth hormone?

No. Growth hormone signals the liver and other tissues to make IGF-1. IGF-1 LR3 is a modified copy of IGF-1 itself, so it acts one step further down the chain and does not raise growth hormone.

Has IGF-1 LR3 been tested in humans?

Not in published clinical trials. Its effects are known from cell studies and animal research, plus the broader science on IGF-1.

What does LR3 stand for?

Long R3. Long refers to the 13 amino acids added to the front of the molecule, and R3 refers to arginine, abbreviated R, replacing the amino acid at position 3.

Is IGF-1 LR3 a peptide?

It is usually called one, but at 83 amino acids it is closer to a small protein. Scientists often put the line between peptides and proteins at around 50 amino acids.

IGF-1 LR3 Peptide: What It Is, Research and Side Effects

Name: IGF-1 LR3: Muscle Growth & Recovery
Uploaded: 2024-04-08T07:18:07-07:00
Duration: 11 min 43 s
Channel: Matthew Farrahi
Description: Matthew explains what IGF-1 LR3 is, how it differs from natural IGF-1, how it acts on muscle cells, and the pros, cons and open questions in the research.

IGF-1 LR3

Also called Long R3 IGF-1, LR3 IGF-1, Long arginine 3 IGF-1

From Matthew Farrahi’s Peptide Insights research · Updated October 28, 2026

IGF-1 LR3 (Long R3 IGF-1) is a lab-made, modified version of insulin-like growth factor 1, the hormone that carries out much of growth hormone's work in the body. Its changes make it bind far less to the proteins that normally hold IGF-1 in check, so more of it stays active. It is widely used to grow cells in labs, but there are no published human clinical trials of it.

Type

Modified IGF-1, 83 amino acids (a small protein)

Origin

Human IGF-1 with one amino acid swapped and 13 added

Main established use

Cell culture and biomanufacturing in labs

Researched for

Cell and muscle growth, tissue growth in animals

Evidence so far

Cell and animal studies only; no human trials

For education and research only. Not medical advice, and not a recommendation to use anything.

What is IGF-1 LR3?

IGF-1 is a 70-amino-acid hormone made mainly by the liver when growth hormone signals it. IGF-1 LR3 is an engineered version with two changes. The amino acid at position 3 is switched from glutamic acid to arginine, which is where the R3 in the name comes from, and a 13-amino-acid extension is added to the front, which is the Long part. The result has 83 amino acids, larger than most compounds called peptides, so strictly it is a small protein.

It was developed by researchers studying how IGF binding proteins control IGF-1 activity, and it became a standard ingredient in cell culture media, where it helps cells grow and survive in labs and in the manufacture of biologic drugs. The IGF-1 used as a medicine is the natural form, called mecasermin, given to children with severe primary IGF-1 deficiency, not LR3.

How does IGF-1 LR3 work?

Most IGF-1 in the blood is held by a family of IGF binding proteins, which control how much reaches tissues. The changes in IGF-1 LR3 sharply reduce this binding, so more of it stays free to act on the IGF-1 receptor, the same receptor natural IGF-1 uses. Activating that receptor signals cells to take up glucose and amino acids, build protein and divide. IGF-1 also has insulin-like effects on blood sugar. It does not work by raising growth hormone. If anything, extra IGF-1 tends to lower the body's own growth hormone release through feedback.

What has IGF-1 LR3 research looked at?

Cell growth in labs: Its best documented use is in cell culture, where it supports the growth of many cell types more strongly than native IGF-1, because binding proteins made by the cells do not block it.
Muscle cells: Lab studies on muscle cells reported increased protein building and cell growth, in line with what is known about IGF-1 signaling.
Growth in animals: Rat studies reported stronger growth effects than native IGF-1, including on the gut.
Human use: There are no published human clinical trials of IGF-1 LR3. Claims about muscle gain or fat loss in people come from anecdote and from what is known about IGF-1 in general.

The general biology of IGF-1 is well studied, and so is IGF-1 LR3's role as a lab tool. What is missing is any controlled human research on this specific molecule, including how it behaves in the human body and what repeated exposure does.

IGF-1 LR3 vs IGF-1 DES

Both are modified forms of IGF-1 that escape IGF binding proteins, but in opposite ways. IGF-1 LR3 is longer than natural IGF-1 and is often described online as longer lasting, though in rat studies it was cleared from the blood faster than natural IGF-1, most of which is held by binding proteins. IGF-1 DES is shorter, missing the first three amino acids, and is thought to act quickly and briefly near where it is released. Neither has been studied in human trials, so comparisons of their effects in people rest on theory and anecdote rather than data.

IGF-1 LR3 side effects and cautions

Because there are no human trials, the side effects of IGF-1 LR3 in people are not well documented. The closest guide is natural IGF-1 used as a medicine, where low blood sugar is the main known risk. IGF-1 also signals cells to grow and divide, and observational studies have linked higher IGF-1 levels with a higher risk of some cancers, so a current or past cancer is a major caution. Anyone with health questions about it should talk to a qualified clinician.

Low blood sugar: IGF-1 has insulin-like effects, and low blood sugar is the main side effect seen with IGF-1 medicines. People with diabetes or a history of low blood sugar face the most risk.
Cancer: IGF-1 signaling helps cells grow, and its role in tumor growth is an active area of research. This is a major concern for anyone with active cancer or a history of cancer.
Effects seen with IGF-1 medicines: With natural IGF-1 medicines, reported effects include headache, raised pressure inside the skull, enlarged tonsils and local fat changes where it is given. Whether these apply to LR3 is unknown.
Unknown long-term effects: No one has studied what repeated exposure does in humans, including effects on the body's own growth hormone and IGF-1 system, or safety in pregnancy.