Don't Take T3 If You Aren't Hormonally Deficient
Even in that situation, you would also need to verify via blood work that you have an inability to sufficiently convert T4 into T3.
The first thing you're going to get prescribed when you have insufficient thyroid production is most likely Synthroid (the most commonly prescribed brand of T4).
In some rare instances, you may be a poor converter of T4 to T3, which is the active thyroid hormone, the hormone we care about.
If you can prove that to your physician, then in that scenario, you would probably get a prescription of T3 if your doctor isn't completely incompetent.
Often times, these prescriptions would just be a bandaid for an underlying issue causing your deficient thyroid production, or subpar conversion rate though.
Something as simple as a Selenium deficiency can prevent you from converting T4 to T3 in your body properly [R].
Sometimes it can be a lot more complicated though and be related to an autoimmune reaction to something so obscure that you need to play detective and thoroughly learn how your body works in order to find the root cause.
Staving Off Fat Loss In A Calorie Surplus With T3?
For recreational use, T3 during a bulk doesn't serve a productive purpose.
There are arguments on internet forums that go back and forth about how T3 increases protein synthesis, keeps you leaner, and so on.
At the end of the day, T3 is not tissue specific and doesn't discriminate when it comes to burning through stored energy.
It doesn't care if it's ripping through muscle or stored fat, and it will gladly tap into your stored muscle protein and put your body in a catabolic state if you aren't careful.
If you're trying to eat in a calorie surplus and you're stuffing your body with nutrient-dense foods to reach a very specific caloric allotment each day, taking additional T3 at a supraphysiological dose is just eating away at that calorie surplus (and potentially muscle tissue) for no additional benefit.
You're basically just wasting the food, so to speak.
The only purpose T3 serves is if you have insufficient thyroid function that is preventing your metabolism from working properly during that bulk.
In that scenario you would use T3.
However, if you have adequate T3 production in the first place, you don't need additional T3 to keep you lean while you're in the offseason.
You're still going to get fat on T3 no matter how much you take if you eat a sh*tty diet, or you're insulin resistant.
I would actually argue that using T3 during a bulking phase could actually hinder your physique more than help it, as you're limiting the very muscle accrual process that will be what determines how fast your metabolism is at rest to begin with.
The more muscle you have, the more fat you burn without even moving your body.
The more you hinder the muscle growth process, the harder you're making it on yourself to prime your metabolism for getting shredded, and an even worse potential scenario could be that you end up in a worse position than you started by eating away at your muscle and slowing your metabolism down as a result.
Should You Use T3 While On GH (Growth Hormone)?
High doses of GH for prolonged periods of time may downregulate your thyroid production.
It's commonly assumed in the bodybuilding community that you need to take T4 with your GH.
In actual practical application, it often doesn't seem to be the case.
However, there are some individuals who do experience a downregulating effect while on GH.
If you experience the latter scenario, you would be wise to explore exogenous T4 replacement, and then your body would convert what it needs from that T4 into T3 to reach an adequate level of active thyroid hormones.
The only situation on GH in which you would need to use T3 instead of T4 during a bulk would be if you're an suboptimal converter of T4 into T3.
In my opinion, T3 only serves a useful purpose in a bulk if you're a poor converter of T4 to T3, and you actually need it as a result of:
A. A genetic predisposition
B. Using other drugs that are suppressing your thyroid hormone production
C. An autoimmune disease that you can't figure out how to treat (I strongly advise you find the root of this ASAP instead of masking the symptoms with exogenous thyroid hormones)
I'm a big fan of sporadic T3 use for cutting, but not for bulking unless you are hormonally deficient.