What the label lists and what reports show
Levothyroxine side effects
Levothyroxine's FDA label lists its adverse reactions in its own words. FAERS holds 300,456 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is fatigue (25,385 reports).
- Generic listed
- Boxed warning
Check it at the source: the levothyroxine FDA label on DailyMed (effective February 20, 2024, AbbVie Inc.); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of LEVOXYL?LEVOXYL can cause serious side effects, including: heart problems. You may experience an increased heart rate, chest pain and irregular heartbeat. Your risk of developing heart problems may be greater if you are elderly, have heart problems, or if you take too much LEVOXYL. Your healthcare provider may reduce your dose or stop treatment with LEVOXYL for a while if you develop heart problems. worsening diabetic control. If you are a diabetic, it may be harder to control your blood sugar levels causing hyperglycemia while taking LEVOXYL. Check your blood sugar levels closely after starting, changing, or stopping treatment with LEVOXYL, and tell your healthcare provider if there are any changes. Your healthcare provider may have to change your diabetes treatment plan. weak or brittle bones. […]
Source: FDA drug label (LEVOXYL), Pfizer Laboratories Div Pfizer Inc, effective December 15, 2020 (SPL set 758588c3-c63e-491b-0aa2-4f50d80cb174), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsAdverse reactions associated with SYNTHROID therapy are primarily those of hyperthyroidism due to therapeutic overdosage [see Warnings and Precautions ( 5 ) and Overdosage ( 10 ) ]. They include the following: General: fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating Central nervous system: headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia Musculoskeletal: tremors, muscle weakness, muscle spasm Cardiovascular: palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest Respiratory: dyspnea Gastrointestinal: diarrhea, vomiting, abdominal cramps, elevations in liver function tests Dermatologic: hair loss, flushing, rash Endocrine: decreased bone mineral density Reproductive: menstrual irregularities, impaired fertility Seizures have been …
Source: FDA drug label, AbbVie Inc., effective February 20, 2024 (SPL set 1e11ad30-1041-4520-10b0-8f9d30d30fcc), via openFDA; the full label on DailyMed
What do FAERS reports show?
300,456 reports list levothyroxine among the medicines taken, each a report of something that happened, not proof the medicine caused it; 19,275 are coded as a death and 85,074 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 23 reactions below, 11 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Levothyroxine: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions