Thyroid Drugs

Question 1

Synthetic T4

Answer 1

• Treatment for hypothyroid
• Prohormone, T1/2: 7 days, shortened T1/2 w/ thyrotoxicosis & P450 inducers
• 1x/day dosing based on body weight

Question 2

Synthetic T3

Answer 2

• Limited Use
• Hormone, T1/2: 1 day, shortened T1/2 w/ thyrotoxicosis & P450 inducers
• Multiple daily doses based on body weight

Question 3

KI

Answer 3

• KI tablets or saturated KI solution
• Tx for hyperthyroid to achieve Wolff-Chaikhoff effect (excess I- blocking thyroid function)
• Escape from WC after a few days of tx

Question 4

Radioactive Iodine

Answer 4

• Kills thyroid
• Tx for Grave’s, thyroid nodules or caner
• Outpatient tx

Question 5

Thionamides: Propylthiouracil & Methimazone

Answer 5

• Anti Thyroid Drugs
• Inhibit TPO catalyzed reactions and block I- organification
• PTU has a shorter T1/2 than Methimazole (preferred for less frequent dosing)
• PTU inhibits 5’-deiodinase which inhibits peripheral deiodination of T4 –> T3 vs. Methimazole has minimal effect on deiodination
• Methimazole always preferred EXCEPT in Pregnancy– crosses placenta causing aplasia of newborn
  Adverse Effects:
• Maculopapular pruritic Rash (most common)
• Hepatitis- more common w/ PTU, can be fatal
• Asymptomatic transaminase elevations
• Agranulocytosis (0.1-0.5% pt)
  ~50% cross-sensitivity btwn. PTU and methimazole so switching isn’t recommended if pt. has a bad reaction to one of them