Retatrutide Phase 3 Update — Diabetes and Osteoarthritis Trials Hit Primary Endpoints

Retatrutide — the first-in-class triple hormone receptor agonist developed by Eli Lilly — has been one of the most closely watched compounds in the metabolic research space since its Phase 2 results were published in the New England Journal of Medicine in 2023. Those Phase 2 results were historic: a 24.2% mean body weight loss at the 12 mg dose over 48 weeks, exceeding the comparable data from tirzepatide’s SURMOUNT-1 trial.

Two recent Phase 3 readouts now begin to test whether the Phase 2 signal holds at the larger scale required for FDA submission. Here’s what the TRANSCEND-T2D-1 and TRIUMPH-4 trials show — and what comes next.

A Quick Refresher on Retatrutide

Retatrutide is a once-weekly subcutaneous peptide that activates three different receptors simultaneously: glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon. The triple-receptor design distinguishes it from tirzepatide, which is a dual GIP/GLP-1 agonist, and from semaglutide, which targets GLP-1 alone.

The glucagon receptor activation is the meaningful mechanistic addition. It engages hepatic energy expenditure and lipolytic pathways that the dual and single agonists don’t reach — adding a third lever to the metabolic intervention beyond the incretin-pathway effects that drive weight loss in semaglutide and tirzepatide.

The Phase 3 program for retatrutide is divided across two trial families. TRIUMPH covers obesity and obesity-adjacent indications. TRANSCEND-T2D evaluates retatrutide specifically in type 2 diabetes. Together, the programs span multiple sub-trials targeting weight, glycemic control, osteoarthritis pain, sleep apnea, cardiovascular and renal outcomes, and metabolic liver disease.

TRANSCEND-T2D-1 — First Type 2 Diabetes Phase 3 Hits All Endpoints

In March 2026, Eli Lilly announced positive topline results from TRANSCEND-T2D-1, the first Phase 3 trial evaluating retatrutide specifically in adults with type 2 diabetes. The trial enrolled patients with a mean diabetes duration of 2.5 years who had not achieved adequate glycemic control through diet and exercise alone.

The trial met its primary endpoint and all key secondary endpoints at the 40-week assessment:

• A1C reduction of up to 2.0% from baseline (using the efficacy estimand)
• Nearly 17% body weight loss at the same time point
• No weight loss plateau observed through 40 weeks — the trajectory was still trending downward at the trial endpoint

The 17% weight loss figure is particularly notable in a T2D population. Earlier-generation GLP-1 therapies typically delivered 6-10% weight reduction in T2D trials, and tirzepatide’s SURPASS program in T2D showed weight reductions in the 11-15% range. The retatrutide T2D data approaches the weight-loss magnitude previously seen only in obesity-specific trials.

TRIUMPH-4 — Obesity Plus Knee Osteoarthritis

Earlier, in December 2025, Lilly reported topline results from TRIUMPH-4, which evaluated retatrutide in adults with obesity or overweight and knee osteoarthritis. The trial tested the two highest investigational doses (9 mg and 12 mg) over the trial duration and reported across both weight and pain endpoints.

Key results from the higher-dose arm:

• Average weight loss of 71.2 lbs
• WOMAC pain score reduction of up to 4.5 points (75.8%)
• 58.6% of participants achieved at least 25% weight loss
• 23.7% achieved at least 35% weight loss
• More than 1 in 8 retatrutide-treated patients reported complete absence of knee pain at the end of the trial
• Significant improvements in physical function measurements

Knee osteoarthritis is a meaningful comorbidity in the obese population. Chronic mechanical loading of the joint accelerates cartilage degradation, and weight reduction is one of the few non-surgical interventions that consistently improves symptoms. The TRIUMPH-4 results suggest retatrutide’s weight loss may translate into substantial functional improvement for patients with co-occurring obesity and osteoarthritis, potentially delaying or avoiding total joint replacement in some cases.

What’s Next — Seven Additional Readouts Expected in 2026

The TRANSCEND-T2D-1 and TRIUMPH-4 readouts are early markers in a much larger Phase 3 program. Eli Lilly has stated that seven additional Phase 3 trial readouts are expected to complete in 2026, spanning:

• Additional sub-trials within the TRIUMPH obesity program (maintenance dosing, registration-tier trials)
• Additional T2D sub-trials within the TRANSCEND program
• Moderate-to-severe obstructive sleep apnea
• Chronic low back pain
• Cardiovascular and renal outcomes
• Metabolic dysfunction-associated steatotic liver disease (MASLD)

This is a broader Phase 3 footprint than tirzepatide had at the equivalent program stage, and it suggests Lilly is positioning retatrutide as a multi-indication intervention rather than a narrowly-targeted obesity drug. Several of these sub-trials would individually represent regulatory submissions on their own merit.

For regulatory timing, Lilly has not publicly committed to a Phase 3 to approval submission timeline. Industry analysts have speculated based on tirzepatide’s precedent — which moved from Phase 3 to FDA approval in approximately 18 months — that a retatrutide submission could come in late 2026 or 2027 if the Phase 3 data continues to track the Phase 2 efficacy signal.

Our Take

The Phase 3 data so far supports the dramatic Phase 2 efficacy signal. A 17% weight loss in TRANSCEND-T2D-1 and 71.2 lbs in TRIUMPH-4 are both consistent with retatrutide retaining its leading position in weight-loss efficacy across the GLP-1 family. What’s strategically more interesting is the multi-indication Phase 3 scope — Lilly is testing retatrutide against a clinical surface broader than typical for a metabolic compound, which would position it as a category-redefining therapeutic if the broader program continues to deliver.

For the research community, retatrutide remains the most-discussed compound in the GLP-1 family right now and is widely available through research-use-only channels at varying purity and verification standards. For full mechanism background, current research applications, and discount codes for vetted vendors, see our Retatrutide profile and Discount Codes page.