Levothyroxine vs Methimazole: side effects in FDA adverse event reports
At a glance
| Levothyroxine | Methimazole | |
|---|---|---|
| Drug class | – | Thyroid Hormone Synthesis Inhibitor |
| Brand names | Levothyroxine Sodium | – |
| FAERS reports | 300,326 | 7,802 |
| Serious | 59% | 71% |
| Death outcome | 19,275 | 641 |
| Boxed warning | Yes NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS Thyroid hormones, including levothyroxine sodium, either alone or with other therapeutic agents, should not be u | No |
Signals both drugs share
| Reaction | Levothyroxine | Methimazole | Label screen (A / B) |
|---|---|---|---|
| Hyperthyroidism | 1,321 · PRR 6.6 | 407 · PRR 77.8 | Labeled — boxed warning / Likely indication confounding |
| Goitre | 420 · PRR 7.5 | 50 · PRR 33.1 | Not in label — unassessed / Not in label — unassessed |
| Thyroidectomy | 446 · PRR 23.3 | 10 · PRR 15.7 | Likely indication confounding / Likely indication confounding |
| Thyroid mass | 283 · PRR 5.3 | 24 · PRR 17.1 | Likely indication confounding / Likely indication confounding |
Labeled signals distinctive to Levothyroxine
- Hypothyroidism — n=4,422, PRR 10.7 Labeled — boxed warning
- Atrial tachycardia — n=407, PRR 15.8 Labeled (related wording)
- Pruritus genital — n=383, PRR 13.0 Labeled (related wording)
Labeled signals distinctive to Methimazole
- Agranulocytosis — n=259, PRR 38.2 Labeled adverse reaction
- Anti-neutrophil cytoplasmic antibody positive vasculitis — n=25, PRR 27.8 Labeled (related wording)
- Dysmorphism — n=18, PRR 20.5 Labeled adverse reaction
- Choanal atresia — n=16, PRR 407.7 Labeled adverse reaction
- Insulin autoimmune syndrome — n=12, PRR 97.3 Labeled adverse reaction
- Aplasia cutis congenita — n=10, PRR 205.4 Labeled (related wording)
Common questions
Which has more reported side effects, Levothyroxine or Methimazole?
Levothyroxine has 300,326 FAERS reports and Methimazole 7,802. More reports does not mean more dangerous: volume follows how widely and how long a drug has been used, prescription vs over-the-counter status, and publicity. The seriousness share (59% vs 71%) is a better like-for-like signal of the kind of problems reported.
Which side effects do Levothyroxine and Methimazole share?
Both drugs carry disproportionality signals for hyperthyroidism, goitre, thyroidectomy, thyroid mass. Shared signals often reflect a shared drug class or the condition both are used for.
Is Levothyroxine or Methimazole safer?
FAERS cannot answer that: it has no denominator and no comparator. Use this page to see which risks are already on each label and which signals are unlabeled, then check the prescribing information and your clinician for your situation.
Sources: openFDA FAERS pair statistics (OpenPV build 2026-09-14), FDA labels via openFDA, openFDA count queries for seriousness and deaths. Pair chosen because it is a commonly searched comparison. Generated 2026-10-08.