Medicine & USMLE

Levothyroxine (T4) vs Liothyronine (T3)

Other Endocrine Pharm
  1. Thionamides (PTU vs Methimazole)
  2. ADH Antagonists (Conivaptan, Tolvaptan)
  3. Levothyroxine (T4) vs Liothyronine (T3)
  4. Demeclocycline
  5. Desmopressin
  6. Cinacalcet
  7. Sevelamer


Liothyronine and levothyroxine are synthetic analogs of thyroid hormones. Liothyronine is synthetic T3, while levothyroxine is synthetic T4. Both drugs act by replacing endogenous thyroid hormones in the body, and thus help to treat hypothyroidism. Overly high doses of these thyroid-replacement drugs include thyrotoxicosis, which describes overactive thyroid hormone, which classically presents with atrial fibrillation.

Key Points

  • Levothyroxine (synthetic T4) and Liothyronine (synthetic T3)
    • Mechanism:
      • Replace endogenous thyroid hormones
      • Levothyroxine is less active and has longer half-life than liothyronine
      • Liothyronien is more active and has shorter half-life
    • Clinical Use:
      • Hypothyroidism
      • Myxedema
      • May be abused for weight loss
    • Adverse Effects:
      • Thyrotoxicosis
        • Atrial fibrillation
        • Arrhythmia
        • Tachycardia
        • Heat intolerance
        • Increased appetite
        • Dyspnea
        • Heat intolerance
        • Tremors