Levothyroxine vs Liothyronine: side effects in FDA adverse event reports
At a glance
| Levothyroxine | Liothyronine | |
|---|---|---|
| Drug class | – | – |
| Brand names | Levothyroxine Sodium | Liothyronine Sodium |
| FAERS reports | 300,326 | 8,654 |
| Serious | 59% | 45% |
| Death outcome | 19,275 | 205 |
| 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 | Yes NOT FOR TREATMENT OF OBESITY OR FOR WEIGHT LOSS Thyroid hormones, including liothyronine sodium, either alone or with other therapeutic agents, should not be us |
Signals both drugs share
| Reaction | Levothyroxine | Liothyronine | Label screen (A / B) |
|---|---|---|---|
| Hypothyroidism | 4,422 · PRR 10.7 | 430 · PRR 33.1 | Labeled — boxed warning / Labeled — boxed warning |
| Rhinitis allergic | 460 · PRR 7.6 | 264 · PRR 153.1 | Not in label — unassessed / Not in label — unassessed |
| Autoimmune thyroiditis | 540 · PRR 11.1 | 52 · PRR 33.9 | Likely indication confounding / Likely indication confounding |
| Polycystic ovaries | 257 · PRR 13.0 | 263 · PRR 487.8 | Not in label — unassessed / Not in label — unassessed |
| Thyroidectomy | 446 · PRR 23.3 | 16 · PRR 22.8 | Likely indication confounding / Likely indication confounding |
Labeled signals distinctive to Levothyroxine
- Hyperthyroidism — n=1,321, PRR 6.6 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 Liothyronine
- None stood out among the strongest labeled signals.
Common questions
Which has more reported side effects, Levothyroxine or Liothyronine?
Levothyroxine has 300,326 FAERS reports and Liothyronine 8,654. 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 45%) is a better like-for-like signal of the kind of problems reported.
Which side effects do Levothyroxine and Liothyronine share?
Both drugs carry disproportionality signals for hypothyroidism, rhinitis allergic, autoimmune thyroiditis, polycystic ovaries, thyroidectomy. Shared signals often reflect a shared drug class or the condition both are used for.
Is Levothyroxine or Liothyronine 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.