T3 and T4 thyroid hormones comparision

T3 (Liothyronine sodium) and T4 (Levothyroxine sodium) are both thyroid hormones, but they differ in potency, half-life, and how the body uses them.

T4 (Levothyroxine sodium)

• Full name: Levothyroxine sodium (synthetic thyroxine)
• Role: A prohormone — it is converted in the body into T3, the active form
• Potency: Much weaker than T3
• Half-life: Long (~7 days) → provides stable thyroid levels
• Medical use: Standard treatment for hypothyroidism
• Athlete use: Rarely used alone, sometimes stacked with T3 for longer thyroid regulation

T3 (Liothyronine sodium)

• Full name: Liothyronine sodium (synthetic triiodothyronine)
• Role: The active thyroid hormone — directly increases metabolism and fat burning
• Potency: About 3–4x stronger than T4
• Half-life: Short (~24 hours) → fast acting
• Medical use: Used when T4 alone isn’t effective
• Athlete use: Preferred in cutting cycles for rapid fat loss, but comes with higher risk of muscle loss and thyroid suppression

Key Differences

✅ In short:

• T4 = storage hormone (slow, weaker, stable)
• T3 = active hormone (fast, strong, burns fat)

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