Thyroid Drugs

Thioamides

• Drugs 
  • propylthiouracil (PTU)
  • methimazole
• Mechanism
  • inhibit thyroid hormone synthesis by blocking peroxidase-catalyzed reactions 
    • iodination of tyrosine residues of thyroglobulin
    • coupling of DIT and MIT
  • propylthiouracil ↓ peripheral conversion of T₄ to T₃
• Clinical use
  • HYPERthyroidism
• Toxicity 
  • skin rash
  • agranulocytosis (rare) 
  • aplastic anemia
  • hepatotoxicity (PTU)
    • PTU not used first line in non-pregnant adults and children for this reason
• Pregnancy
  • use PTU in the first trimester 
• methimazole crosses the placenta and enters breast milk (may be teratogenic)

Thyroid hormones

• Drugs
  • levothyroxine
    • synthetic T₄ (thyroxine)
  • triiodothyronine
    • T₃
• Mechanism
  • replaces endogenous thyroid hormones
• Clinical use
  • HYPOthyroidism
  • myxedema
• Toxicity
  • thyrotoxicosis
    • heat intolerance
    • tachycardia
    • dyspnea
    • ↑ appetite
    • tremors
    • arrhythmias