Methimazole and propylthiouracil are two of the main medication options to help manage hyperthyroidism. These medications work by inhibiting the body’s production of thyroid hormones. More specifically, it blocks the oxidation of iodine in the thyroid gland. This ultimately leads to a reduction in triiodothyronine (T3). I can tell you from clinical practice that methimazole is used much more frequently than propylthiouracil. I’ll provide a comparison of methimazole versus propylthiouracil (PTU) in this article and why you are more likely to see methimazole.
The adverse effect profile is always a major consideration when comparing two medications. When looking at methimazole versus propylthiouracil, it is hard to ignore the boxed warning associated with PTU. PTU carries a boxed warning for hepatotoxicity and this is certainly a major reason why we would avoid its use in favor of methimazole. This would make a great board exam nugget!
Kinetics and ease of administration is another important differentiating factor. Methimazole has a significantly longer half-life. PTU only has a half-life of around 1 hour which is going to necessitate more frequent dosing and may impact quality of life and patient adherence. Propylthiouracil needs to be dosed multiple times per day (usually 2-3 times per day). Methimazole’s half-life is in the range of 4-6 hours and we will likely be able to get away with once daily dosing in most situations. This can be a huge determining factor for patients when selecting methimazole versus PTU.
Why Use PTU Over Methimazole?
The major situation where you would consider PTU over methimazole is in the 1st trimester of pregnancy. There will always be a risk/benefit calculation when using medications in pregnancy but if medication therapy for hyperthyroidism is determined to be necessary, the safety data in the first trimester leans in favor of PTU versus methimazole. There will obviously be specialty involvement in the situation of pregnancy and hyperthyroidism.
When comparing the thionamides of methimazole and PTU, hepatotoxicity is a major reason why methimazole is more popular. Patient adherence is another strong consideration in favor of methimazole. Looking for more on thyroid hormones? Check out this podcast on levothyroxine.
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