What did @daviddemesquita actually say?
The short version: T3 should not be used without T4, high-dose T3 is dangerous and counterproductive, and reverse T3 is mostly misunderstood. He also claimed that anabolics and growth hormone suppress free T4 more than free T3 in labs he's reviewed.
He pushed back on a coach who bragged about using T3 instead of T4, calling that stance "dumb." His core argument is that T4 acts as a reservoir that converts to T3 and reverse T3, creating a physiological buffer. He also flagged seeing T3 doses as high as 100-150 micrograms in prep athletes, describing the result as excess adrenaline output and insomnia. He acknowledged thyroid downregulation during caloric restriction as a real and expected phenomenon.
Does the science back this up?
Mostly, yes. The physiology here is well-established, and his basic framework is correct. Where things get shakier is the clinical nuance around reverse T3 and the blanket dismissal of standalone T3 use.
T4 (levothyroxine) is the standard first-line treatment in hypothyroidism management, per guidelines from the American Thyroid Association (Jonklaas et al., 2014, Thyroid). T4 is a prohormone that undergoes peripheral deiodination to active T3, primarily via type 1 and type 2 deiodinase enzymes. This conversion happens in the liver, kidneys, and muscle tissue. Reverse T3 (rT3) is a metabolically inactive isomer produced from the same T4 pool, and its clinical relevance is genuinely contested. Citing rT3 as benign in most people is consistent with mainstream endocrinology, though some integrative practitioners dispute this. The claim that caloric restriction suppresses thyroid output is well-supported. Studies on dieting athletes consistently show drops in T3 and sometimes T4, driven partly by reduced leptin signaling (Rosenbaum et al., 2010, Journal of Clinical Endocrinology and Metabolism).
What did they get wrong (or right)?
He got the big picture right, but the claim that reverse T3 is "very rare" to see elevated deserves scrutiny. He also oversimplifies why T3-only protocols exist in clinical settings.
Elevated reverse T3 is actually documented in a range of physiological stressors including sepsis, prolonged fasting, and surgery (Chopra, 1997, European Journal of Endocrinology). In the context of aggressive bodybuilding prep, with caloric restriction, high training volume, and often concurrent stimulant use, rT3 elevation is not as rare as he suggests. That said, its functional significance even when elevated remains debated.
On the T3-only question: there are legitimate clinical scenarios where liothyronine (T3) is used standalone or in combination, particularly in patients with poor T4-to-T3 conversion or those who remain symptomatic on T4 monotherapy (Idrees et al., 2020, Journal of the Endocrine Society). His dismissal of T3-only approaches as universally wrong overcorrects. The bodybuilding context he's describing, where coaches are running 100-150 micrograms of T3, is a separate and genuinely reckless practice that deserves the criticism he gives it. Those doses exceed typical replacement ranges by three to five times and carry real cardiac and bone-density risks.
What should you actually know?
If you are using thyroid hormones outside of a supervised medical relationship, you are taking on serious risks that no prep coach is qualified to manage. These are not optimization supplements. They are regulated hormones with narrow therapeutic windows.
Supraphysiologic T3 use in athletes has been associated with muscle catabolism, cardiac arrhythmia risk, and suppression of endogenous thyroid axis function. The irony the creator points to is real: athletes using high-dose T3 to accelerate fat loss may be burning lean tissue they worked months to build. Thyroid suppression during prolonged caloric restriction is a physiological adaptation, not a malfunction. Glandulars, as he notes, have limited efficacy when the body is already downregulating output in response to energy deficit. The mechanism driving thyroid suppression in prep is largely neuroendocrine, not a substrate deficiency that a glandular supplement can override. Any thyroid protocol in a competitive athlete warrants baseline labs, ongoing monitoring of free T3, free T4, TSH, and potentially rT3, and physician oversight. This is not a domain where coaching credentials substitute for a medical license.
Bottom line
This video is more accurate than most thyroid content circulating in fitness spaces. The creator correctly identifies the T3-only hype as physiologically naive and correctly points to caloric restriction as a primary driver of thyroid downregulation in prep athletes. The oversimplification of reverse T3 and the framing of T3-only use as never appropriate are the main weak points. The broader warning against supraphysiologic T3 dosing is sound and worth repeating.