Nutrition
Thyroid Hormones T3 and T4
Thyroid hormones T3 and T4 set cellular metabolic speed—labs and symptoms together explain energy and training response better than calories alone.
Thyroid hormones T4 (thyroxine) and T3 (triiodothyronine) circulate in blood and act on cells to control heat production, protein turnover, and how quickly you burn fuel at rest and during exercise. T4 is the main output; tissues convert a portion to the more active T3.
Low or high levels change heart rate, sweat response, and session RPE before scale weight moves much.
Common questions
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Is T3 always tested with T4?
Panels vary—many screens start with TSH and free T4; free T3 is added when symptoms and history warrant. Interpret with your clinician, not reference ranges alone.
Related terms
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