What Happens If You Took Two Doses of Synthroid by Mistake
A single accidental double dose of levothyroxine (Synthroid) is generally well-tolerated and rarely causes serious harm in most adults, though you should monitor for symptoms and skip the next dose if it occurs on the same day.
Immediate Effects and Expected Symptoms
The good news is that acute levothyroxine overdose typically produces minimal to mild symptoms, even with substantial ingestions. The drug has a long half-life (approximately 7 days), so a single double dose represents only a modest increase in your total body thyroid hormone load.
Most people will experience no symptoms at all from accidentally doubling a single dose. When symptoms do occur, they are typically minor and self-limited 1, 2.
Possible Symptoms (if any occur):
- Cardiovascular: Palpitations, increased heart rate, mild increase in blood pressure
- Neurological: Nervousness, anxiety, tremors, headache, hyperactivity
- General: Heat intolerance, sweating, insomnia
- Gastrointestinal: Diarrhea, abdominal cramps, nausea
- Other: Flushing, feeling jittery or "wired"
These symptoms, if they appear, usually develop within 6-24 hours and resolve on their own without treatment 1, 2.
What You Should Do
Immediate Actions:
- Do not panic - serious complications from a single double dose are extremely rare
- Skip your next scheduled dose if the double dose occurred on the same day, then resume your normal schedule
- Do not try to "make up" for the extra dose by skipping multiple future doses
- Stay well-hydrated
When to Seek Medical Attention:
Contact your physician or seek medical care if you develop:
- Chest pain or angina (especially if you have pre-existing heart disease)
- Severe palpitations or irregular heartbeat
- Significant shortness of breath
- Severe tremors or agitation
- Altered mental status or confusion
Special caution for high-risk patients: If you have underlying cardiovascular disease, are elderly, or are on high doses of levothyroxine (>200 mcg/day), you should contact your physician for guidance even with mild symptoms 1.
Why This Usually Isn't Dangerous
The FDA labeling emphasizes that levothyroxine has a narrow therapeutic index, meaning the difference between therapeutic and toxic doses is relatively small 1. However, this primarily applies to chronic overtreatment, not acute single-dose errors.
Key protective factors:
- The long half-life means blood levels rise gradually, not suddenly
- Your body has regulatory mechanisms that buffer against acute thyroid hormone fluctuations
- Most adverse effects from levothyroxine overtreatment develop with sustained elevation over weeks to months 3
Research on accidental pediatric ingestions found that even children who consumed large amounts of levothyroxine (far more than a double dose) experienced only minor, self-resolving symptoms in 27% of cases, with no serious outcomes 2.
Chronic Overtreatment vs. Acute Double Dose
It's important to distinguish between a single accidental double dose and chronic overtreatment. The serious complications mentioned in guidelines—cardiovascular effects (atrial fibrillation, angina), bone loss, and metabolic disturbances—occur with sustained overtreatment over months to years, not from a single dosing error 3, 1.
The guidelines note that 20% of patients on thyroid hormone are chronically overtreated, and these patients face increased risks of:
- Atrial fibrillation (especially elderly)
- Accelerated bone loss (especially postmenopausal women)
- Increased cardiovascular events
- Mood disturbances
These risks do not apply to your single accidental double dose 3.
Rare Severe Cases
While extremely uncommon, massive intentional overdoses (50-100+ times the therapeutic dose) have caused severe symptoms including coma, though even these cases typically resolve with supportive care 4. Your accidental double dose represents nowhere near this level of exposure.
Prevention Going Forward
To avoid future dosing errors:
- Use a pill organizer to track daily doses
- Take your levothyroxine at the same time each day
- If you're unsure whether you took your dose, it's safer to skip that day than to risk a double dose
- Consider setting a daily alarm or reminder
Bottom Line
An accidental double dose of Synthroid is a common medication error that rarely causes significant problems. Monitor yourself for mild symptoms of excess thyroid hormone, but understand that most people experience no symptoms at all. Resume your normal dosing schedule the next day, and contact your physician if you develop concerning cardiovascular or neurological symptoms, particularly if you have pre-existing heart disease 1.