T3 a misunderstood fat-burner

T3: a misunderstood fat-burner?

T3 (triiodothyronine) is a naturally-occurring thyroid hormone that regulates metabolism, digestion, cognition, immunity and even sexual function to a small extent.

This is a hormone that we all have and need in order to function optimally, but it can also be used exogenously to replace endogenous T3 levels in people with hypothyroidism, or as a fat-burning drug in a bodybuilding context.

The exogenous form of T3 is called liothyronine (brand name Cytomel) and is orally bioavailable. Even though this drug is safe and effective when it comes to fixing hypothyroidism, it can be very dangerous when used to speed up fat loss in a bodybuilding context.

What does T3 do?

Exogenous T3 essentially does the same as endogenous T3, meaning that its function is to increase basal metabolic rate, promote the production of new mitochondria, increase the size of lymphocytes (which improves immunity), improve cognitive function by regulating the central nervous system, protect the heart by causing vasodilation (even though hyperthyroidism can be harmful to the heart), improve blood sugar levels and insulin sensitivity and to support muscular function.

The benefits of T3 as a PED

Chances are you don’t really care about the benefits of using T3 therapeutically, and what you really want to know are the benefits of using T3 as a PED for fat-loss. Here’s what you can expect from taking exogenous T3 as a bodybuilder:

  • T3 will essentially destroy any fat loss plateau resulting from bad metabolism.
  • T3 will allow you to get under a bodyfat percentage that would simply be unattainable if you were only relying on your normal metabolism.
  • T3 will easily get rid of the stubborn fat that the body never seems to want to get rid of.

The side-effects of T3

Unfortunately, these benefits come with a hefty price:

  • T3 does not discriminate between fat or muscle mass, meaning that it will cause catabolism and it will burn muscle UNLESS you are also running anabolics with T3.
  • T3 will make you sweat a lot more.
  • T3 may cause anxiety.
  • T3 may accelerate your heart rate.
  • T3 may cause you to feel lethargic and frail.
  • T3 will increase your body temperature.
  • T3 will increase your appetite, which is counterproductive when trying to lose fat.
  • T3 will make it harder for you to get a pump at the gym.
  • T3 will suppress your TSH (Thyroid-Stimulating Hormone), meaning that it will stop your natural T3 production (similar to how SARMs & AAS suppress Testosterone).

T3 protocol for bodybuilders

When it comes to T3, the dose that you take makes a HUGE difference in terms of side-effects. T3 is often sold in 25mcg tabs, and that is the dose you should get started with. Take 25mcg a day for up to 2 weeks, then increase to 50mcg/day. If you see that at 50mcg the benefits don’t get much better but the side-effects become more dangerous, simply go back to 25mcg. If you can tolerate 50mcg/day well, go up to 75mcg a day after two weeks of being on 50mcg a day.

Some people go even higher, but I personally think that you can get all the fat loss you need from 75mcg, even just 50mcg.

There isn’t a standard cycle length you should stick to, you can theoretically be on a pretty strong T3 dose for months on end, but if you use T3 properly (meaning that you take it when you are fairly lean and want to get leaner as opposed to taking it to going from fat to not-fat), you won’t need to use it for more than 8 weeks at a time.

As I mentioned before, T3 will suppress your endogenous TSH levels and thus your endogenous T3. There is no such thing as a PCT for your thyroid hormones, but these tend to bounce back in 1 to 3 weeks. In other words, for up to 3 weeks after you come off T3, your metabolism will be fucked up and gaining a ton of fat will be easy if you are not careful with your food intake.

In order to mitigate the side-effects of T3, you should drink a ton of water and keep your electrolytes balances, and you should simply use the highest effective dose at which the side-effects don’t ruin your life. This will take some trial and error, but you will be fine.

In terms of timing, you want to take it in the morning and preferably fasted to maximize its effectiveness. The half-life is of 2.5 to 3 days, so you can theoretically take it any other time of the day. Needless to say, T3 is not a magic pill and it will require serious dieting, exercise and cardio to provide optimal results.

What about T2 and T4?

T2 and T4 are precursors to T3 that can also be used exogenously to burn fat. Instead of being active T3, they convert into T3 inside the body. Both are less suppressive than T3 (they may not even suppress you at all), but they are less powerful because they must convert into the active hormone. Both are much safer than regular T3, and they can be acquired legally without a prescription in many countries.

A common T2 dose is 200 to 400mcg a day, and T4 is usually dosed at 2-3x the regular T3 dose.

In my opinion, you are better off using T3 if you want consistent results.


Even though T3 is quite hardcore, it is way more manageable than something like DNP or Clenbuterol. In my opinion, using T3 to go from 20% body fat to 12% bf is dumb, because you can do that naturally without having to endure any side-effects, but using T3 to go from say 10-12% to 8% or below makes sense. If used reasonably, T3 is a viable fat loss agent for recreational and professional bodybuilders alike.


William Davis

William has been studying and experimenting with bodybuilding pharmacology for over 6 years. After being an independent researcher for all these years, he has decided to share his knowledge with the bodybuilding community through his science-based articles. His approach to enhanced bodybuilding can be summed up in the saying “less is more”, as he believes that prioritizing harm mitigation and looking for ways to maximize the positives is the key to longevity in bodybuilding.