T3 — Triiodothyronine

T3 binds thyroid hormone receptors in cell nuclei, activating gene expression that controls basal metabolic rate. Increases mitochondrial activity, oxygen consumption, and heat production. Directly regulates protein synthesis, carbohydrate metabolism, and lipid metabolism. Supraphysiological T3 dramatically accelerates metabolism — leading to significant fat loss but also muscle catabolism.

Not a steroid. No ester. Natural amino acid derivative — tyrosine with three iodine atoms. Oral tablet form (Cytomel). Short-to-medium half-life of approximately 2.5 days.

T3 is used in performance settings to dramatically accelerate metabolic rate during cutting phases. The thyroid hormone controls the speed of virtually every metabolic process in the body and supraphysiological T3 produces fat loss that exceeds what is achievable through caloric restriction and training alone. Competitive bodybuilders use it in the final weeks of contest prep when maximum leanness is the goal. It is also used to counteract the metabolic slowdown that occurs during prolonged caloric restriction, where endogenous thyroid output naturally decreases as a protective response to energy deficit.

T3 is most commonly combined with Clenbuterol in cutting protocols for additive metabolic acceleration. Because T3 at performance doses causes significant muscle catabolism, it is almost never used without anabolic support, meaning it typically appears in stacks that include Testosterone, Trenbolone, or other compounds specifically to offset the tissue-wasting effects. This creates a situation where multiple high-risk compounds are used simultaneously, each compensating for a side effect of another, and the total risk profile of the stack is substantially greater than any single component.

• Hypothyroidism — primary treatment
• Thyroid cancer — post-thyroidectomy suppression of TSH
• Combination therapy with T4 in refractory hypothyroid cases
• Myxedema coma — IV T3 administration

• Thyroid suppression — potentially permanent with prolonged supraphysiological dosing
• Cardiac arrhythmias — atrial fibrillation, tachycardia
• Angina and heart failure with pre-existing cardiac disease
• Muscle catabolism — breakdown of muscle tissue at supraphysiological levels
• Bone density loss with chronic use
• Anxiety, tremors, insomnia — sympathomimetic effects
• Heat intolerance, excessive sweating
• Hypothyroid rebound — when discontinued abruptly

The risk of permanent thyroid suppression from exogenous T3 use is real and the probability increases with dose and duration. The thyroid axis does not always fully recover to baseline function after exogenous hormone use, particularly when the suppression has been prolonged or severe. Muscle catabolism at supraphysiological doses is significant and the anabolic compounds typically stacked alongside T3 are included specifically to mitigate this effect, but mitigation is not elimination. Cardiac arrhythmia risk, particularly atrial fibrillation, is a serious concern at elevated doses. The fact that T3 is used to offset metabolic adaptation during a diet creates a cycle where users need progressively more intervention to maintain the same rate of fat loss.

Challenging to detect directly — T3 is endogenous. WADA testing uses TSH suppression and thyroid hormone panel analysis.