Does taking T3 burn only fat, or can it cost muscle too?

T3 is an active thyroid hormone that affects metabolism throughout the body. Replacing a documented deficiency is different from pushing thyroid activity above physiological needs to lose weight. The Cytomel label explicitly warns against using thyroid hormone for obesity or weight loss and lists muscle weakness among the effects of excessive exposure.[1]

A faster metabolism is not a guarantee that lost weight is exclusively fat. Excess thyroid action can also produce palpitations, tremor, heat intolerance, sleep problems, and other signs of hyperthyroidism. Adding an anabolic steroid does not establish that muscle or the cardiovascular system is protected from all consequences of thyroid excess.[1]

Fatigue during a diet does not diagnose hypothyroidism or establish a need for T3. Thyroid results should be interpreted alongside symptoms, medication history, and the clinical setting. For someone with genuine thyroid disease, treatment decisions belong to that diagnosis; for physique goals, a falling scale number alone is too narrow a measure of benefit.[1]