Liothyronine
most data
The Dose Guy’s verdictSep 1, 20262 sources
Liothyronine works. It’s actual thyroid hormone, and adding it to a cut raises your metabolic rate, burns more calories at rest, and moves the scale. The problem is that it doesn’t pick what it burns: muscle goes alongside the fat, your own thyroid production dials down while you run it, and the recovery period after you stop is slower than you want it to be. If you pair it with something that protects lean mass and you titrate carefully, it’s a real tool. If you run it carelessly, you come out lighter but softer, with a sluggish thyroid that takes weeks to wake back up.
What it is
Synthetic triiodothyronine, the active thyroid hormone your body normally converts from T4, prescribed for hypothyroidism and borrowed by the cutting community because it does exactly what the label says: it raises metabolic rate. Every cell in your body has thyroid receptors, and when you push T3 above your natural level, the furnace runs hotter across the board. That’s the appeal and the problem. The metabolic increase is indiscriminate. Fat oxidation goes up, but protein turnover goes up too, and your hypothalamic-pituitary-thyroid axis reads the elevated T3 and dials your own production down. You get a faster cut up front and a slower metabolism on the back end until your axis recovers.
The bodybuilding community has run liothyronine for decades, almost always stacked with anabolics to offset the muscle cost. On its own, the net effect is weight loss with a composition trade-off that most people don’t like once they see it in the mirror. GC-1 is the research-stage attempt to solve this problem, a thyroid agonist that targets the liver’s beta receptor and spares the rest, but it has zero human data.
What the research shows
Good human data, just not for the use case you’re thinking of. The best numbers come from thyroid-replacement trials, not fat-loss protocols, but they show the metabolic signal clearly:
Realistic protocols
Start low and titrate by feel, not by calendar. Liothyronine hits fast, peaks in about two hours after a dose, and that sharp spike is why most people split it through the day rather than taking one dose. Take it on an empty stomach, away from calcium and iron, which blunt absorption. The axis suppression is real from day one, so the exit plan matters as much as the dose.
The standard cutting run is 6 to 8 weeks. Start at 25 mcg per day, split into two doses. If the scale is moving and sides are manageable, hold at 25. Push to 50 only if the effect stalls, not on a calendar. Some people go to 75 or even 100, but the muscle cost accelerates fast above 50 and the return on extra T3 starts to flatten. When you finish, taper down rather than stopping cold, dropping 12.5 mcg every few days, because your own axis needs time to wake back up and a hard stop leaves you sluggish.
The community almost universally stacks liothyronine with something anabolic, testosterone at minimum, because without it the mirror outcome is disappointing: you lose weight, but you lose shape with it. Protect your protein intake too. If you’re not willing to run an anabolic alongside it, the honest advice is that the incretins (tirzepatide, retatrutide) do a better job of preserving lean mass while cutting.
Side effects
Real drug, real side profile. Most of it is predictable from what thyroid hormone does:
