Bodybuilders will sometimes utilize the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) when trying to cut, or reflect more pronounced physical transformation.
T3 and T4 are known to “accelerate fat loss” while on a cut or diet, but as key thyroid hormones – it is extremely important to know what you’re doing before you go tinkering around with supplemental T3/T4. We will explain an ideal T3/T4 combination dosage for bodybuilding, what these key thyroid hormones do, and their role in physique.
Thyroid Hormones and the HPA
T3 and T4 are initially produced in the thyroid gland. This organ is responsible for regulating the metabolism of nutrients from food (fats, carbs, protein), CNS function, cholesterol, libido, menstrual cycles, CNS, muscle building, and overall body weight/composition.
The thyroid gland utilizes iodine to produce T3 and T4. However, the thyroid will normally produce much more T4, compared to T3 – typically 50% – 60% more. The majority of T3 within our bodies will come from circulating T4 that is converted to T3. This conversion occurs within tissues like the liver and skeletal muscle.
While more T4 is produced by the thyroid gland, T3 is the ‘stronger’ hormone, as the “biologically active” form that acts on cells and tissues. In perfect homeostasis, these hormones should never be too low or too high – as we see in cases of hypothyroidism or hyperthyroidism.
The actual levels of T3 + T4 in the blood are closely regulated by the connection between the pituitary gland, the thyroid, and the hypothalamus. This is referred to as the “HPA” or hypothalamic pituitary axis. Readers of the blog will recognize this from our previous articles on SARM use or anabolic use impacting the HPTA.
The chain is:
- Hypothalamus produces thyroid releasing hormone (TRH)
- This tells the pituitary to produce more (or less) of thyroid stimulating hormone (TSH)
- Subsequently, this tells the thyroid to produce more or less of T3 + T4
Whenever levels of T3 or T4 are low in the blood, the pituitary will naturally produce TSH to signal the thyroid to increase production of T3/T4 and release.
If the levels of T3 and T4 are instead too high in the blood, the TSH production from the pituitary gland will decrease, signaling to the thyroid to more slowly release T3/T4 in lower concentrations.
This is known as a “negative feedback loop,” which we have also used to describe the HPTA when it comes to testosterone production. These systems help to keep hormone levels within a ‘normal’ or functional range.
The Connection Between T3/T4 and Bodybuilding
Due to the direct relationship T3 and T4 have with metabolism, they are often viewed as a means to alter the way fat, protein and carbohydrates are metabolized by the body.
When there is more T3 and T4 than what is “normal,” people will find it much easier to slim down during a cut or diet – hence their implementation in the realm of bodybuilding.
For those looking to achieve an extremely low body fat percentage (%), exogenous T3 and T4 are commonly used to get lean.
While some forum users will recommend strictly T3, there is importance in taking exogenous T4 along with the T3. If you go into supraphysiological doses of these hormones, it is important to keep both in check, rather than supplementing with a higher dose of an individual one (T3 monotherapy).
T3/T4 Combination Dosage
Typical combination dosage for T3 & T4 will depend upon how you’re administering it. Some individuals may be using Bitiron – which is 50 mcg T4 and 12.5 mcg T3.
This can be slowly titrated upward to be 100 mcg T4 and 25 mcg T3 per day across two divided doses if required. Highly unlikely you would need to go that high, though. Some people may also find it difficult to take larger doses all at one time and instead opt to take it divided through morning and mid-day.
T3 is known to have a stimulant effect when taken exogenously, so we recommend taking it in the morning on an empty stomach.
You do not need to exceed doses of 12.5 mcg of T3, generally speaking, and certainly not 25mcg of T3.
The amount of exogenous T3 or T4 you will require will depend largely on your physiology, which is why it is important to get accurate labs relating to your thyroid hormone panels. The recommended lab work includes Free T4 and Free T3, TSH, Reverse T3, Thyroid Antibodies, Cortisol, and Sex Hormones.
While athletes will typically dose 25 mcg – 50 mcg T3 over the course of an 8 week cycle, studies seem to indicate monotherapy (I.E., just using T3) as less effective than a combined form of T3:T4 medication.
Athletes or bodybuilder will often stick to synthetic T3 because it is more ‘potent,’ but we should not disregard the negative feedback loop. By opting for a combination T3:T4, tapering off the cycle is a little easier and you won’t impact endogenous production to the same degree with high doses of monotherapy.
When using a T3:T4 combination product, it is best to stick to a 1:4 or 1:2 ratio, given that those with a normal functioning thyroid gland typically produce a range of 10 – 20 mcg T3 and 94 – 110 mcg T4 daily.
Bottom Line on T3/T4
It is highly likely that someone looking to drop body fat percentage needs to exceed 50 mcg of T3 – any higher, and you will undoubtedly wind up losing a lot of muscle needlessly by pushing yourself into hyperthryoid territory.
If you are at a point where you need to induce hyperthyroidism before an event or during a cut, you probably failed to properly execute the cut in the first place. T3 is among the most catabolic supplements you could be using when it comes to fat burning.
As such, we recommend starting with a lower dosage, titrating upwards, and ensuring you use a dual T3:T4 regimen. Bodybuilders recommending or running 75 mcg to 125 mcg might seem a little excessive.
Thankfully, the HPA is much more forgiving than the HPTA – there is less concern of your hormones becoming fully suppressed or production ‘shutting down’ after completing an 8-week cycle.