The T3/T4 Trap: How Thyroid Hormones Destroy Your Hard-Earned Muscle

[Type-IIx]

Author​

Cormac Mannion (Type-IIx)

Date​

Friday, October 03 2025

Introduction​

Key Takeaways​

Key Takeaways

1. Exogenous T3 significantly breaks down muscle protein
2. T3 selectively destroys growth-responsive type IIA muscle fibers
3. GH naturally increases peripheral conversion of T4 to T3, which is beneficial
4. The reduction in serum T4 from GH use is typically not problematic for healthy individuals
5. Thyroid hormone supplementation is irrational for bodybuilders as it accelerates muscle loss
6. Exogenous GH cannot cause central hypothyroidism but may reveal pre-existing conditions
7. Thyroid hormone misuse carries serious health risks including cardiac arrhythmias
8. Common bodybuilding practices of using T3 deep into competition prep can be extremely destructive to muscle mass

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Introduction​

Here’s another Q&A format article that was asked by a user in the Team Ampouletude Telegram group and led to a fruitful discussion… and fleeting references to still widespread bad practices among bodybuilders with respect to thyroid hormone use… something so destructive to muscle mass deep into prep when it’s typically applied… commonly with a hand-wave to the effect that “he’s on Halo, he won’t notice any muscle loss…“

Thyroid hormone is potently muscle catabolic. It chews through muscle aggressively and selectively, especially the growth-responsive type II fibers.

Question​

Since exogenous growth hormone (exogenous GH) reduces serum T4, should a healthy bodybuilder supplement exogenous T4 (Levothyroxine; Synthroid; Tiromel) or T3 (Liothyronine; Cytomel) as a rule with exogenous GH?

Definitions​

T3: Triiodothyronine

T3 (exogenous) [Cytomel, LT3]: Liothyronine

T4: Thyroxine

T4 (exogenous) [Tiromel, Synthroid, LT4]: Levothyroxine

T3 in its endogenous (triiodothyronine) and exogenous (liothyronine) form is the actively catabolic hormone among the thyroid hormones (e.g., T2, T3, T4). T4, despite some endogenous effects, for enhanced bodybuilding discussion is effectively the prodrug to the active T3.

fT3; FT3: free T3, the bioactive fraction in blood

RMR: resting metabolic rate

REE: resting energy expenditure

UCP: uncoupling protein

LBM: lean body mass

Dose-Response​

Metabolic Rate​

Given a seventy-five microgram (75 µg) Cytomel dose (daily for 14 days):

• fT3 increased by 1.7-fold
• RMR increased by 15% (adjusted for LBM)
• UCP2 increased 1.7-fold and UCP3 increased 2.4-fold. [1]

A trend was also seen for a direct correlation between REE/LBM and increases in fT3 levels, with an average 0.11 kcal/kg/d increase in REE/LBM for each 10 pg/dL increase in fT3.

Read More on Substack​

Substack | Read​

 

[anon55]

Good to know. I'm hypothyroidic and using Euthyral, a combination of 100ug of T4 and 20ug of T3.

 

[Type-IIx]

Good to know. I'm hypothyroidic and using Euthyral, a combination of 100ug of T4 and 20ug of T3.

If you’re hypothyroid and on this kind of replacement dose then you’re in a different category bro! You need it

 

[anon55]

If you’re hypothyroid and on this kind of replacement dose then you’re in a different category bro! You need it

With it, TSH about 1.15 and t3l at 3.4ng/L before next dose.

 

[TTren]

i ran 1.2gram test 600 tren a and 600 eq and still lost muscle with 75mcg t3 and clen

 

[Amgiji]

I was at
3.61 ulU/mL TSH,
1.6 ng/dL t4 free
3.1 pg/mL t3 free
25.9 ng/dL reverse t3

trying to get more into the "optimal range"
I take 45mg T4 and 60 mcg slow release t3 every day.

they are now
1.11 ulU/mL TSH
0.8 ng/dl t4 free
3.0 pg/mL t3 free
12.0 ng/dL reverse t3

I feel better but, was going that high year-round a bad idea?

 

[Sampei]

@Type-IIx I understand the whole T3 debacle and I agree with it but the T4 I'm a bit confused. You can't convert too much T4 to go hyperthyroid for my understanding, so even if you try to ADD T4 but one doesn't need it, it will go to waste no?

Using HGH you increase the conversion from t4 to t3 so adding t4 will just make you utilize more of it and for my personal experience I never went above the FT3 range, so why is it bad to supplement T4 and ONLY T4 if one doesn't go above the range?

fyi my thyroid is perfectly healthy but during cuts it goes a little be lower then what I'm used too so I was thinking why not supplement a bit of T4 with HGH to keep the t3 level higher

 

[Photon]

@Type-IIx I understand the whole T3 debacle and I agree with it but the T4 I'm a bit confused. You can't convert too much T4 to go hyperthyroid for my understanding, so even if you try to ADD T4 but one doesn't need it, it will go to waste no?

Using HGH you increase the conversion from t4 to t3 so adding t4 will just make you utilize more of it and for my personal experience I never went above the FT3 range, so why is it bad to supplement T4 and ONLY T4 if one doesn't go above the range?

fyi my thyroid is perfectly healthy but during cuts it goes a little be lower then what I'm used too so I was thinking why not supplement a bit of T4 with HGH to keep the t3 level higher

On a cut my rt3 spiked and ft3 dropped.
if anything, i think it makes more sense to add t3 than t4..

 

[Type-IIx]

@Type-IIx I understand the whole T3 debacle and I agree with it but the T4 I'm a bit confused. You can't convert too much T4 to go hyperthyroid for my understanding, so even if you try to ADD T4 but one doesn't need it, it will go to waste no?

Using HGH you increase the conversion from t4 to t3 so adding t4 will just make you utilize more of it and for my personal experience I never went above the FT3 range, so why is it bad to supplement T4 and ONLY T4 if one doesn't go above the range?

fyi my thyroid is perfectly healthy but during cuts it goes a little be lower then what I'm used too so I was thinking why not supplement a bit of T4 with HGH to keep the t3 level higher

It sounds like you're gonna be fine doing this. Still, T4 is risky to combine with GH. The reason it can be particularly risky to supplement T4 on GH is thyrotoxicosis, that section is in the article there near the end. Overt thyrotoxicosis can happen under the conditions that you describe, and if you're on gear too, even more readily. Everyone on gear has suppressed TSH. Everyone on GH has "suppressed" T4 and increased T3, it's just partly how they work.

I know I've heard it all before too bro – that T4 can't make you hyperthyroid – it's just bad broscience. In fact, 1/5 US patients on thyroid hormones is subclinically or overtly hyperthyroid, and the usual treatment is levothyroxine (L-thyroxine) or T4. They get into it in this paper here, I went and looked this up for you on the topic: The problem of exogenous subclinical hyperthyroidism - PubMed

Now, I would never say it's wrong to replace thyroid when on a diet to endogenous levels, like 25 mcg T3 daily or 75 mcg or so of T4, right? But to use it under a misguided belief that it's beneficial for enhanced protein turnover and extra fat-burning? That's where the notion becomes counterproductive, and muscle losses will follow.

But frankly, even replacement will in itself cause some muscle loss, still. This is because the reason the body suppresses thyroid levels and output during caloric restriction (& very low carbohydrate intakes) is to spare muscle protein/anticatabolism… that's the very well supported theory anyway!

 

[Sampei]

It sounds like you're gonna be fine doing this. Still, T4 is risky to combine with GH. The reason it can be particularly risky to supplement T4 on GH is thyrotoxicosis, that section is in the article there near the end. Overt thyrotoxicosis can happen under the conditions that you describe, and if you're on gear too, even more readily. Everyone on gear has suppressed TSH. Everyone on GH has "suppressed" T4 and increased T3, it's just partly how they work.

I know I've heard it all before too bro – that T4 can't make you hyperthyroid – it's just bad broscience. In fact, 1/5 US patients on thyroid hormones is subclinically or overtly hyperthyroid, and the usual treatment is levothyroxine (L-thyroxine) or T4. They get into it in this paper here, I went and looked this up for you on the topic: The problem of exogenous subclinical hyperthyroidism - PubMed

Now, I would never say it's wrong to replace thyroid when on a diet to endogenous levels, like 25 mcg T3 daily or 75 mcg or so of T4, right? But to use it under a misguided belief that it's beneficial for enhanced protein turnover and extra fat-burning? That's where the notion becomes counterproductive, and muscle losses will follow.

But frankly, even replacement will in itself cause some muscle loss, still. This is because the reason the body suppresses thyroid levels and output during caloric restriction (& very low carbohydrate intakes) is to spare muscle protein/anticatabolism… that's the very well supported theory anyway!

Thanks for the in-depth explanation it was something I thought as well but I needed someone else to tell that it was just kinda of a bad idea or better not really needed if one had a healthy thyroid.

What about on a bulk instead? Adding 50/100mcg of T4 (checking bloods to not lower TSH too much, so adjusting the dosage) with HGH to use the increased conversion into T3 in keeping the body less prone in storing fat? Stupid idea? Just bro science as well? Counterproductive for the bulk?

 

[Moulefruit]

So even in a context of a contest prep, you don’t recommend to use t3 or t4 ?
Even with the most anti catabolic steroid AKA Trenbolone in the stack ?

 

[Type-IIx]

So even in a context of a contest prep, you don’t recommend to use t3 or t4 ?
Even with the most anti catabolic steroid AKA Trenbolone in the stack ?

That is my view, yes. Tren is the most anticatabolic steroid but it’s not like it cancels out the fact you took thyroid hormone, you know? It doesn’t block synthroid’s protein breakdown because thyroid hormone’s not an antiandrogen, it breaks down protein by different routes, basically. Tren still has to claw back what it would not have had to otherwise

 

[dinfar1337]

You can't convert too much T4 to go hyperthyroid for my understanding

my free t3 and normal t3 was on the lower range of normal and my free t4 was low, lower than ref range.

hopping on steroids and hgh made it worse.

jumping on t4 i got my numbers back up all above ref range but still on the low end with 120mcg and now im at middle range of normal with 200mcg t4. but i swear tracking my calories my maintenance havent gone up by 1 calorie from it. this thyroid bullshit is scam.

atleast i can be happy about normal numbers now

 

[Stealth500]

Thanks for the in-depth explanation it was something I thought as well but I needed someone else to tell that it was just kinda of a bad idea or better not really needed if one had a healthy thyroid.

What about on a bulk instead? Adding 50/100mcg of T4 (checking bloods to not lower TSH too much, so adjusting the dosage) with HGH to use the increased conversion into T3 in keeping the body less prone in storing fat? Stupid idea? Just bro science as well? Counterproductive for the bulk?

tried to sneak in an extra question huh... LOL. No soup for you.

 

[Slowww]

I found thet cut results without t3 were much better visually, and I definitely looked less stringy and flat by the end.even 12.5-25mcg had a pretty bad visual effect. A bit of t4 still let me recomp and hold weight at very low body fat with a minimal amount of trenbolone. After seeing the difference personally and reading this article Im definitely going to push back if the use is requested, because it really seems counter productive with various forms of evidence. This makes it seem much more sensible that it wasn’t continued in my protocol after the first experience

 

[Sampei]

I found thet cut results without t3 were much better visually, and I definitely looked less stringy and flat by the end.even 12.5-25mcg had a pretty bad visual effect. A bit of t4 still let me recomp and hold weight at very low body fat with a minimal amount of trenbolone. After seeing the difference personally and reading this article Im definitely going to push back if the use is requested, because it really seems counter productive with various forms of evidence. This makes it seem much more sensible that it wasn’t continued in my protocol after the first experience

so no T3, yes T4?

 

[Scottomus]

Seems I need to read more into this. I was led to believe t4 was required when running HGH.

I have been deleting on gh, t3,t4 and Reta and it’s worked tremendously. Albeit I do believe I’ve looked constantly flat and rather stringy

 

[Sampei]

I tried T3 once and said never again. Makes you look like shit, flat, no pumps, and I finished the cut looking worse then I started just leaner but lost a lot.

T4 I have mixed feelings on it. Used it years ago and never did after but lately I was considering it a little bit.

 

[Slowww]

so no T3, yes T4?

Yeah, but just my unscientific opinion i run t4 year round. Cut or bulk doesn’t matter and have been growing much better

 

[Sampei]

Yeah, but just my unscientific opinion i run t4 year round. Cut or bulk doesn’t matter and have been growing much better

Was your thyroid panel ok even before starting it? Just trying to eliminate the chance that you were hypothyroid before starting it

How much T4 you taking?