Thioamides

Question 1

Examples

Answer 1

• PTU
• Methimazole

Question 2

Methimazole and propylthiouracil inhibit what process?

Answer 2

Thyroid hormone synthesis (by inhibiting the organification and coupling of thyroid hormone)

Question 3

Propylthiouracil inhibits which biochemical pathway that methimazole does not?

Answer 3

Both affect thyroid hormone synthesis, but PTU also decreases the peripheral conversion of thyroxine to triiodothyronine by blocking 5′–deiodinase

Memory aid:

• _P_TU
• _P_eripheral conversion

Question 4

What are the known toxicities of methimazole and propylthiouracil?

Answer 4

• Skin rash
• Aplastic anemia
• Agranulocytosis (rare)

Question 5

Use in pregnancy

Answer 5

• PTU used in 1^st trimester of pregnancy (due to methimazole teratogenicity)
• Methimazole used in 2^nd and 3^rd trimesters of pregnancy (due to risk of PTU-induced hepatotoxicity)

Question 6

____ is a possible teratogen (can cause ____)

Answer 6

• Methimazole
• Aplasia cutis

Question 7

What laboratory test should be ordered to avoid a rare but devastating adverse effect when starting a patient on methimazole or propylthiouracil?

Answer 7

Complete blood count, to monitor for aplastic anemia or agranulocytosis