Updated: 6/3/2021

Thyroid Drugs

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Snapshot
  • A 32-year-old woman presents to her primary care physician after 3 months of weight loss, intermittent palpitations, and increased anxiety and is found to have bilateral exophthalmos on exam. Her thyroid-stimulating hormone (TSH) is tested and found to be below the detectable limit. She is diagnosed with Graves disease, and informs her doctor that in the interim she has just discovered that she is pregnant. The physician prescribes her a medication to help return her to a euthyroid state. (Methimazole)
Thyroid Hormones
  • Drugs
    • levothyroxine
      • synthetic T4
      • used in the vast majority of cases
    • triiodothyronine
      • T3
      • used only occasionally, in combination with levothyroxine
  • Mechanism of action
    • replacement of endogenous thyroid hormones
  • Clinical use
    • hypothyroidism
  • Adverse effects
    • very rare unless overdosing occurs
    • iatrogenic thyrotoxicosis
      • signs and symptoms of hyperthyroidism
      • risk of fracture
      • risk of arrhythmia
Thioamides
  • Drugs
    • methimazole
      • first-line thioamide
    • propylthiouracil (PTU)
      • used only in pregnant patients, breastfeeding patients, and patients with drug reaction to methimazole
      • mnemonic: PTU for Pregnant, Puerperal, or Post-methimazole Papules, and Pustules
  • Mechanism of action
    • both
      • ↓ thyroid hormone production by interfering with thyroid peroxidase (TPO), and therefore has the following effects
        • ↓ oxygenation of iodide
        • ↓ iodination of tyrosine to monoiodotyrosine (MIT) and diiodotyrosine (DIT)
        • ↓ coupling of MIT/DIT to for T3 and DIT/DIT to form T4
    • PTU only
      • ↓ peripheral conversion of T4 to T3
      • mnemonic: only PTU works Peripherally
  • Clinical use
    • hyperthyroidism
      • used to achieve euthyroid state prior to definitive treatment with radioiodine therapy or surgery
      • definitive treatment for patients who are not radioiodine or surgical candidates
  • Adverse effects
    • bone marrow suppression 
      • agranulocytosis  
      • aplastic anemia
      • pancytopenia
      • watch closely for signs of infection
      • discontinue medication and draw CBC with differential whenever suspected  
    • skin reactions
    • hepatotoxicity
      • worse with PTU (black box warning)
    • unique to methimazole
      • fetotoxic
      • present in significant concentrations in breast milk

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