Drug side effects
levothyroxine Side Effects
The reactions most often reported to the FDA for levothyroxine (also sold as Synthroid, Levoxyl), with demographics, serious outcomes, and reporting trends.
Data updated 2026-05-15
- 304,465
- FAERS reports
Data from the FDA Adverse Event Reporting System (FAERS), 1997 to 2025. A FAERS report records a temporal association, not proof of cause. See our methodology for how figures are compiled and de-duplicated.
The most commonly reported side effects of levothyroxine come from 304,465 reports to the FDA, broken down here by frequency, patient demographics, and serious outcomes.
This page analyzes 304,465 reports to the FDA Adverse Event Reporting System (FAERS) for levothyroxine (Synthroid): the reactions reported most often, who reported them, serious outcomes such as hospitalization, and year-over-year trends. A FAERS report does not prove the drug caused the effect, and report volume tracks how widely a drug is used, not how likely an effect is for any one person. Educational information, not medical advice.
Total Reports
304,465
Reports Mentioning Death
19,579
6.4% of reports — not proof the medicine caused the death
Hospitalizations
88,368
29.0% of reports
Top Indication
Product Used For Unknown Indication
Most Reported Adverse Reactions
Who Reports Side Effects
Gender Distribution
Age Distribution
Reporting Trend by Year
Reactions in Death Reports
Top reactions reported in 19,579 reports where death was an outcome. These are voluntarily reported and do not establish causation.
| Reaction | Reports |
|---|---|
| DEATH | 7,288 |
| PNEUMONIA | 1,368 |
| DYSPNOEA | 1,316 |
| FATIGUE | 1,226 |
| ASTHENIA | 1,168 |
| DIARRHOEA | 1,092 |
| NAUSEA | 1,048 |
| VOMITING | 924 |
| PAIN | 903 |
| GENERAL PHYSICAL HEALTH DETERIORATION | 859 |
| FALL | 831 |
| DECREASED APPETITE | 802 |
| RENAL FAILURE | 793 |
| SEPSIS | 772 |
| RESPIRATORY FAILURE | 765 |
| CARDIAC ARREST | 754 |
| MALAISE | 736 |
| ANAEMIA | 719 |
| PYREXIA | 716 |
Reactions in Hospitalization Reports
Top reactions in 88,368 reports where hospitalization was an outcome.
| Reaction | Reports |
|---|---|
| DYSPNOEA | 7,990 |
| NAUSEA | 7,480 |
| FATIGUE | 7,224 |
| FALL | 7,097 |
| DIARRHOEA | 6,600 |
| PNEUMONIA | 6,323 |
| ASTHENIA | 6,000 |
| PAIN | 5,982 |
| VOMITING | 5,854 |
| HEADACHE | 5,199 |
| DIZZINESS | 4,857 |
| PYREXIA | 4,621 |
| MALAISE | 4,354 |
| ARTHRALGIA | 3,838 |
| WEIGHT DECREASED | 3,804 |
| HYPERTENSION | 3,651 |
| HYPOTENSION | 3,587 |
| ANAEMIA | 3,554 |
Nearby, Related Medications
What the FAERS Data Reveals About levothyroxine Side Effects
The FDA Adverse Event Reporting System (FAERS) contains 304,465 voluntary reports linked to levothyroxine and its brand equivalents (Synthroid, Levoxyl, Tirosint), spanning 1997 through 2025. Of those, 19,579 (6.4%) listed death as an outcome and 88,368 (29.0%) involved hospitalization. The most common indication reported alongside adverse events was Product Used For Unknown Indication.
Demographic breakdowns help contextualize who is being exposed to the drug. Of reports with known sex, 81% were female and 19% male; age distribution skews toward 45-64, with 70,265 reports in that bracket. The single most reported reaction is fatigue with 25,847 submissions, followed by nausea and headache.
How to read these numbers
FAERS is a signal-detection tool, not a scorecard. Reports are voluntary, and a single case may list multiple suspect drugs, so numbers above should not be read as incidence rates or per-patient risk. Widely prescribed drugs naturally accumulate more reports than niche therapies even when individual risk is low. These aggregates are useful for spotting patterns that merit further pharmacovigilance, not for choosing between medications. This page is for educational reference only and is not medical advice, speak with a licensed clinician about any side-effect concerns.
Drugs With a Similar FAERS Report Volume
Other medications with a total FDA adverse-event report count closest to levothyroxine's 304,465.
Read our methodology - how this data is sourced, computed, and verified.