Updated: 2/2/2018

Thyroid Drugs

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Questions
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Evidence
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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
 

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(M1.EC.13.108) A 30-year-old female complains of heat intolerance, excessive sweating, and recent unintentional weight loss. She denies alcohol, tobacco, and other drug use and does not take any medications. Laboratory analysis reveals elevated serum T4 and decreased serum TSH. You elect to manage the patient’s condition pharmacologically with methimazole. Methimazole interferes with the function of which of the following proteins? Tested Concept

QID: 100409
1

Thyroxine-binding globulin

11%

(5/47)

2

Sex-hormone binding globulin

0%

(0/47)

3

Thyroid peroxidase

77%

(36/47)

4

5’-deiodinase

6%

(3/47)

5

Sodium/iodide symporter

6%

(3/47)

M 1 C

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