Internalize Info of the Week: The Antithyroid Drugs PTU and Methimazole

The first entry in a series of selected topics taken from my personal experience of questions I was asked during the week. The first topic is about two antithyroid drugs, propylthiouracil (PTU) and methimazole. I was asked about the mechanism of action of these drugs and their use in pregnancy.

These drugs are used to treat hyperthyroidism. PTU and methimazole work by blocking the production of both T4 and T3 in the thyroid gland itself. PTU also blocks the peripheral conversion of T4 to T3.

PTU has a black box warning for hepatotoxicity. It is considered a second line treatment for hyperthyroidism and should not be used in place of methimazole. It is, however, the drug of choice during the first trimester of pregnancy. It also may be used in breastfeeding mothers.

Methimazole is the drug of choice for hyperthyroidism in general, and hyperthyroidism during the second and third trimesters of pregnancy. It has a lesser risk of hepatotoxicity than PTU. It should not be used in breastfeeding mothers.

Here is a table summarizing the above information:

	Mechanism of Action	Black Box Warning	Use in Pregnancy	Use in Breastfeeding
PTU	Blocks T4/T3 production and T4 to T3 conversion	Hepatotoxicity	First trimester	May be used
Methimazole	Blocks T4/T3 production	-	Second/Third trimester	Should not be used

Some very basic but important facts to remember! Look good out there on the rounds.

Sources:
West-ward Pharmaceutical Corp. "Propylthiouracil Official FDA Information, Side Effects and Uses." Drugs.com | Prescription Drug Information, Interactions & Side Effects. Drugs.com, Aug. 2011. Web. 05 Nov. 2011. <http://www.drugs.com/pro/propylthiouracil.html>.
Par Pharmaceutical Inc. "Methimazole Official FDA Information, Side Effects and Uses." Drugs.com | Prescription Drug Information, Interactions & Side Effects. Drugs.com, Nov. 2009. Web. 05 Nov. 2011. <http://www.drugs.com/pro/methimazole.html>. 
"Propylthiouracil Use During Pregnancy and Breastfeeding | Drugs.com." Drugs.com | Prescription Drug Information, Interactions & Side Effects. Drugs.com. Web. 05 Nov. 2011. <http://www.drugs.com/pregnancy/propylthiouracil.html>.