T2, Fat Burning & Brown Fat: How It Work...

Does increasing fat burning automatically mean losing more body fat? And what would we need to measure to know whether an intervention preserves muscle?

In this Friday solocast, I'm revisiting my conversation with Dr. Amie Hornaman to unpack the research on T2, a lesser-known thyroid hormone derivative, and connect the mechanisms to the body composition questions we care about.

We go inside the mitochondria to explore how fatty acids enter, how our cells regulate fuel use, and how mitochondrial uncoupling releases energy as heat. I also revisit the brown fat research and potential connections with ketogenic metabolism.

Along the way, I clarify several findings from the interview—including what "50% less fat mass" meant in the rat study, why lower body weight does not establish fat-only loss, and why changes in muscle-cell metabolism do not prove muscle preservation.

The T2 research is intriguing, but human evidence remains very limited. This recap explores what the findings suggest, what they cannot establish, and the practical steps we can take now to support muscle and metabolic health.

BACK-TO-SCHOOL SALE!

For a limited time, save 40% sitewide at ketogenicgirl.com with code BACKTOSCHOOL40, including the Protein-Sparing Modified Fasting Library, ToneLUX Red Light Therapy Collection, and Tone Breath Ketone Analyzer.

In this episode, I discuss:

• What T2 is and how it relates to T3 and T4.
• What the 2005 rat study actually found about fat accumulation.
• The carnitine shuttle and how fatty acids reach the mitochondria.
• How AMPK, ACC, and malonyl-CoA help regulate fat oxidation.
• Mitochondrial uncoupling, ATP production, and heat generation.
• Brown fat thermogenesis and why an animal's thyroid status matters.
• Possible connections with keto—and why shared pathways do not establish added benefits.
• The difference between burning dietary fat and losing stored body fat.
• The two-participant human T2 report and Amie's uncontrolled 14-woman project.
• What stronger human research would need to measure.
• Thyroid safety questions and the importance of appropriate prescribed treatment.
• Lean mass versus skeletal muscle during weight loss.
• Practical ways to prioritize protein, resistance training, and consistency.
• Questions to bring to your clinician if you have persistent symptoms.

Research discussed

• Lanni and colleagues, 2005: T2, fat accumulation, and liver fatty acid oxidation in high-fat-fed rats.
  Read the study
• Lombardi and colleagues, 2009: T2, skeletal muscle fatty acid oxidation, AMPK signaling, and thermogenesis.
  Read the study
• Antonelli and colleagues, 2011: Preliminary human report on resting metabolic rate and body weight in two participants.
  Read the study
• Cavallo and colleagues, 2013: Rapid liver-cell responses to T2 in hypothyroid rats.
  Read the study
• Lombardi and colleagues, 2015: Brown fat thermogenesis in hypothyroid rats.
  Read the study

Additional research and resources

• Luukkonen and colleagues, 2020: A short human study of ketogenic eating, liver fat, and hepatic mitochondrial metabolism.
  Read the study
• Hall and colleagues, 2015: A controlled human study illustrating the distinction between increased fat oxidation and net body-fat loss.
  Read the study
• Longland and colleagues, 2016: Protein and exercise during an energy deficit.
  Read the study
• Padron and colleagues, 2014: T2 and thyroid suppression in rats.
  Read the study
• Jonas and colleagues, 2015: T2, thyroid regulation, and potential cardiac effects in mice.
  Read the study
• American Thyroid Association: Thyroid treatment and testing information.
  Thyroid hormone treatment | Thyroid function tests

Listen to the full conversation with Dr. Amie Hornaman in the podcast feed.

Thank you so much for spending this time with me! If you enjoyed the episode, please share it with a friend who loves understanding the science behind metabolism and body composition.

Disclosure and medical disclaimer

Dr. Amie Hornaman sells the T2-containing supplement discussed in the original interview. This recap is educational and does not endorse or recommend purchasing or taking it. Supplemental T2's long-term safety has not been established, and some animal studies have raised concerns about thyroid suppression and heart effects.

Do not start T2, substitute it for prescribed treatment, or change your thyroid medication based on this episode. Consult a qualified healthcare professional about your individual circumstances.