levothyroxine vs propylthiouracil
Side-by-side comparison of levothyroxine and propylthiouracil Data from FDA drug databases (Orange Book, NDC Directory, recalls, shortages) covering 20,000+ approved drugs, plus CMS pricing; see our methodology.
Synthroid, Levoxyl, Tirosint
PTU
Levothyroxine is a medicine that replaces a hormone normally made by your thyroid gland. It is used when the thyroid doesn't make enough hormone on its own.
Propylthiouracil (PTU) is a medicine that treats an overactive thyroid. It helps to lower the amount of thyroid hormone your body makes.
This medicine treats hypothyroidism, a condition where the thyroid gland doesn't make enough thyroid hormone. It can be used in adults and children, even newborns. Levothyroxine can also be used after surgery and radioiodine therapy for thyroid cancer to help lower thyroid-stimulating hormone (TSH) levels.
PTU treats hyperthyroidism, which is when your thyroid gland makes too much thyroid hormone. It is used for Graves’ disease or toxic multinodular goiter when you cannot take methimazole, or when surgery or radioactive iodine is not a good option for you. PTU can also help control hyperthyroidism symptoms before thyroid surgery or radioactive iodine treatment.
Levothyroxine provides a synthetic form of thyroxine (T4), which is the main hormone produced by the thyroid gland. Your body converts T4 into triiodothyronine (T3), the active form of the hormone. By providing T4, levothyroxine helps restore normal thyroid hormone levels in your body.
PTU works by stopping your thyroid gland from making too much thyroid hormone. It does this by interfering with the process that creates these hormones. This helps to reduce the symptoms of hyperthyroidism.
- • Fatigue
- • Increased appetite
- • Weight loss
- • Feeling hot
- • Headache
- • Skin rash
- • Hives
- • Nausea
- • Vomiting
- • Stomach upset
- Tiredness 25,847
- Feeling sick to your stomach 22,021
- Medicine not working 20,559
- Head pain 18,798
- Loose stools 18,178
- The drug is not working 189
- Using the drug for a condition it's not approved for 149
- Overactive thyroid 135
- Baby exposed to the drug during pregnancy 113
- Feeling sick to your stomach 87
Thyroid hormones, including levothyroxine, should not be used for weight loss or to treat obesity. Using high doses can cause serious and life-threatening side effects, especially when taken with stimulant medicines.
PTU can cause severe liver problems, including liver failure, which can be fatal. You should only use PTU if you cannot take methimazole and surgery or radioactive iodine are not right for you. PTU may be the best choice if you need an antithyroid drug during or just before the first 3 months of pregnancy.
Tell your doctor if you are pregnant or plan to become pregnant. Your levothyroxine dose may need to be adjusted during pregnancy. Levothyroxine passes into breast milk, but it is not expected to harm the baby.
PTU may be used during the first 3 months of pregnancy if an antithyroid drug is needed. Talk to your doctor about the risks and benefits of taking PTU while pregnant or breastfeeding.
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How to Read This levothyroxine vs propylthiouracil Comparison
levothyroxine is classified in the Thyroid Hormone drug class, while propylthiouracil sits within the Anti-Thyroid Agent class. Drugs from different classes work through distinct mechanisms, so a head-to-head comparison illustrates trade-offs rather than equivalence. Both drugs are prescription-only, so a licensed provider must authorize use.
Adverse event totals above are pulled from the FDA's Adverse Event Reporting System (FAERS). For these top-ranked reactions alone, levothyroxine has 105,403 submissions while propylthiouracil has 673. Those figures reflect cumulative reporting volume — not per-patient risk — so older, widely dispensed drugs typically look worse on count alone. No direct interaction between these two drugs is listed in our FDA-derived dataset, though co-prescription still warrants pharmacist review. Serious warnings, pregnancy guidance, and contraindications can differ even when indications overlap.
A table cannot substitute for clinical judgment. Effectiveness, tolerability, drug-drug interactions with your other medications, kidney and liver function, pregnancy status, insurance formulary, and price all feed into a decision that only a licensed prescriber can make responsibly. Data here is sourced from FDA Structured Product Labels (SPL) and FAERS, both of which update as manufacturers and clinicians submit new information. This page is for educational purposes only, is not medical advice, and should not be used to self-switch between levothyroxine and propylthiouracil — always consult your physician or pharmacist first.
Important: This comparison is for informational purposes only. Drug effects vary between individuals. Always consult your doctor or pharmacist for personalized medical advice.