Propylthiouracil

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Propylthiouracil (PTU): a quick, easy guide

What it is and what it’s for
- PTU is a medicine used to treat overactive thyroid (hyperthyroidism), including Graves’ disease and toxic multinodular goiter.
- It can be used in a thyroid crisis because it may work faster than some other drugs.
- It’s taken by mouth.

How it works
- PTU lowers thyroid hormone in two ways: it blocks thyroid peroxidase (reducing hormone production) and it blocks the conversion of T4 to the more active T3.
- It does not stop iodine from entering thyroid cells.

When it’s used
- PTU is often a second-line option, used when methimazole, surgery, or radioactive iodine aren’t possible.
- It’s an older drug and not usually the first choice for most patients.

How it is processed in the body
- About 80–95% is absorbed after taking by mouth.
- It concentrates in the thyroid.
- It has a short plasma half-life (about 1–2 hours), but its effects in the thyroid may last, so dosing can be every 8 hours or longer.
- Full normalization of thyroid levels can take 2–4 months.

Pregnancy considerations
- PTU is category D (fetal risk). In the first trimester it may be preferred over methimazole to reduce organ-development risks, but later in pregnancy methimazole is often preferred to avoid liver problems in the mother.
- Transplacental passage can cause mild hypothyroidism in the fetus near term, usually resolving after birth.

Common and serious side effects
- Common: skin rash, itching, hair loss, swelling of the salivary glands, nausea, vomiting, heartburn, taste changes, joint or muscle pains, headache.
- Serious but less common: very low white blood cell counts (agranulocytosis), low platelets (thrombocytopenia), and serious liver injury or liver failure.
- In 2009, the FDA warned about the risk of serious liver injury with PTU. Because of this, PTU is often not the first choice in non-pregnant adults or children.
- Rarely, near the end of pregnancy, a baby may develop a goiter or mild hypothyroidism that typically improves after birth.

Other notes
- PTU has been used since the 1940s and is listed as an essential medicine by the World Health Organization.
- It is generally well tolerated, with side effects occurring in about 1 in 100 people.