levothyroxine vs methimazole
PlainMeds reviewed this comparison against our methodology in August 2026. It places FDA and CMS record fields side by side, without recommending either medication.
Side-by-side comparison of levothyroxine and methimazole. 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
Tapazole
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.
Methimazole (Tapazole) is a medicine that lowers the amount of thyroid hormone your body makes. It helps treat hyperthyroidism (overactive thyroid).
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.
This medicine treats hyperthyroidism, a condition where your thyroid gland makes too much thyroid hormone. Too much thyroid hormone can cause weight loss, fast heartbeat, sweating, and feeling nervous or irritable. Methimazole helps to reduce these symptoms by lowering the amount of thyroid hormone in your body.
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.
Methimazole stops the thyroid gland from making thyroid hormone. It does this by blocking an enzyme needed to produce the hormones T3 and T4. This helps to bring thyroid hormone levels back to normal.
- • Fatigue
- • Increased appetite
- • Weight loss
- • Feeling hot
- • Headache
- • Nausea
- • Headache
- • Fatigue
- • Rash
- • Joint pain
- Fatigue 25,847
- Nausea 22,026
- Headache 18,797
- Diarrhoea 18,176
- Dyspnoea 16,216
- Fatigue 426
- Hyperthyroidism 384
- Nausea 372
- Diarrhoea 325
- Headache 315
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.
Methimazole can cause serious side effects. Talk to your doctor right away if you develop a fever, sore throat, or mouth sores, as these could be signs of a serious blood problem. This medicine may also harm your liver.
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.
Talk to your doctor if you are pregnant or plan to become pregnant. Methimazole can harm your unborn baby. It is also important to discuss breastfeeding with your doctor while taking this medicine.
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How to Read This levothyroxine vs methimazole Comparison
levothyroxine is classified in the Thyroid Hormone drug class, while methimazole 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 101,062 submissions while methimazole has 1,822. 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 represent individual circumstances. Data here is sourced from FDA Structured Product Labels (SPL) and FAERS, both of which update as manufacturers and clinicians submit new information. This registry does not provide medical, treatment, substitution, or medication-change directions for levothyroxine or methimazole.
Registry boundary: This comparison displays source-record fields and does not provide individualized medical direction.