What should a patient with hypothyroidism do if they accidentally take 400 micrograms (mcg) of levothyroxine, twice their prescribed dose of 200 mcg?

Immediate Management of Accidental Levothyroxine Overdose

If a patient accidentally doubles their levothyroxine dose from 200 mcg to 400 mcg on a single occasion, they should skip the next scheduled dose and resume their regular 200 mcg dose the following day, while monitoring for symptoms of thyrotoxicosis over the next several days. 1

Why This Single Overdose Is Not an Emergency

• A single doubled dose of levothyroxine (400 mcg instead of 200 mcg) is unlikely to cause serious harm due to the medication's long half-life of 6-7 days, which means blood levels change gradually rather than acutely 1
• The FDA label notes that symptoms of overdose "may not necessarily be evident or may not appear until several days after ingestion" because levothyroxine takes time to be absorbed and metabolized 1
• Most acute levothyroxine overdoses resolve independently without medical intervention, as documented in case reports 2

Specific Instructions for the Patient

• Skip the next scheduled dose (the day after the doubled dose) to compensate for the extra 200 mcg taken 1
• Resume the regular 200 mcg dose on the second day after the overdose 1
• Do not attempt to reduce subsequent doses beyond skipping one dose, as this can cause unnecessary fluctuations in thyroid hormone levels 3

Symptoms to Monitor Over the Next 7-10 Days

• Watch for signs of thyrotoxicosis including rapid or irregular heartbeat, chest pain, tremors, excessive sweating, heat intolerance, nervousness, irritability, or sleeplessness 1
• Monitor for gastrointestinal symptoms such as diarrhea or vomiting 1
• Be aware that confusion and disorientation can occur with overdose, though this is more common with larger or repeated overdoses 1

When to Seek Medical Attention

• Contact a healthcare provider immediately if the patient experiences chest pain, severe palpitations, shortness of breath, or signs of cardiac arrhythmia 1
• Seek emergency care if confusion, disorientation, or altered mental status develops 1
• For elderly patients or those with pre-existing cardiac disease (coronary artery disease, atrial fibrillation, heart failure), a lower threshold for medical evaluation is warranted due to increased risk of cardiac complications 3, 4

Important Caveats for Specific Populations

• Elderly patients (>60 years) are at higher risk for atrial fibrillation and cardiac complications even with therapeutic doses, so any symptoms of palpitations or chest discomfort warrant immediate evaluation 4, 5
• Patients with known coronary artery disease should be monitored more closely, as even minor over-replacement can unmask or worsen cardiac ischemia 4
• Pregnant women should contact their healthcare provider for guidance, as thyroid hormone requirements are carefully titrated during pregnancy 1, 5

What NOT to Do

• Do not induce vomiting or take activated charcoal unless specifically directed by poison control, as the medication has already been absorbed by the time symptoms would appear 1
• Do not reduce the regular dose below 200 mcg for subsequent days (beyond skipping one dose), as this creates unnecessary fluctuation 3
• Do not panic or seek emergency care for an asymptomatic single doubled dose in an otherwise healthy adult 2

Follow-Up Considerations

• No change in the regular monitoring schedule is needed for a single accidental doubled dose 3
• If symptoms of thyrotoxicosis develop and persist, contact the prescribing physician for potential temporary dose reduction and earlier TSH monitoring 1
• The next scheduled TSH check should proceed as planned (typically 6-8 weeks after any dose change, or annually if stable) 3

Contrast with Chronic Overtreatment

• This guidance applies specifically to a single accidental doubled dose, which is fundamentally different from chronic overtreatment 3
• Chronic levothyroxine overtreatment (TSH <0.1 mIU/L maintained over time) significantly increases risks for atrial fibrillation, osteoporosis, fractures, and cardiovascular mortality, but a single doubled dose does not carry these long-term risks 3, 6, 4
• Approximately 25% of patients on levothyroxine are unintentionally maintained on excessive doses chronically, which is a separate clinical problem requiring dose adjustment 3