From the FDA Drug Label
The signs and symptoms of overdosage are those of hyperthyroidism [see Warnings and Precautions (5)and Adverse Reactions (6)] . In addition, confusion and disorientation may occur Cerebral embolism, shock, coma, and death have been reported. Seizures occurred in a 3-year-old child ingesting 3. 6 mg of levothyroxine. Symptoms may not necessarily be evident or may not appear until several days after ingestion of levothyroxine sodium. Reduce the levothyroxine sodium dosage or discontinue temporarily if signs or symptoms of overdosage occur. Initiate appropriate supportive treatment as dictated by the patient’s medical status For current information on the management of poisoning or overdosage, contact the National Poison Control Center at 1-800-222-1222 or www.poison. org.
If a patient accidentally takes two doses of their levothyroxine, reduce the levothyroxine sodium dosage or discontinue temporarily if signs or symptoms of overdosage occur. It is also recommended to initiate appropriate supportive treatment as dictated by the patient’s medical status. For current information on the management of poisoning or overdosage, contact the National Poison Control Center at 1-800-222-1222 or www.poison.org 1.
From the Research
If a patient accidentally takes two doses of levothyroxine, they should monitor for symptoms of excess thyroid hormone but generally don't need to skip the next dose. For a one-time double dose, serious adverse effects are unlikely since levothyroxine has a long half-life (about 7 days) and takes time to build up in the system. The patient should watch for symptoms like rapid heartbeat, nervousness, anxiety, tremors, headache, or insomnia. If severe symptoms occur, they should contact their healthcare provider or seek medical attention. Patients with heart conditions should be particularly cautious and may want to consult their provider even without symptoms. For the subsequent scheduled dose, they should resume their normal dosing schedule without skipping or adjusting doses unless directed otherwise by their healthcare provider. This approach works because the body adjusts to small fluctuations in thyroid hormone levels, and a single extra dose typically won't cause significant harm in most patients, as suggested by the principles outlined in a study on medication adherence and dosing strategies 2. It's also worth noting that while beta-blockers have been used to manage symptoms of hyperthyroidism, such as those that might occur with excessive levothyroxine intake 3, 4, 5, the primary concern in this scenario is monitoring for and managing potential symptoms of excess thyroid hormone, rather than immediately adjusting medication or adding new medications without healthcare provider guidance. Additionally, the treatment of hypothyroidism with levothyroxine, with or without additional thyroid hormones, does not directly inform the management of an accidental double dose but does underscore the importance of careful dosing and monitoring in thyroid hormone replacement therapy 6.