Can Levothyroxine Cause Seizures?
Levothyroxine can cause seizures, but only in the context of acute overdose or toxicity—not at therapeutic replacement doses. This is a rare but documented complication that occurs when excessive thyroid hormone creates a hypermetabolic state affecting neurological function.
Seizures from Levothyroxine Overdose
The FDA drug label explicitly states that seizures have occurred following levothyroxine overdose, specifically documenting a case in a 3-year-old child who ingested 3.6 mg 1. This represents a critical threshold, as pediatric case reports confirm seizures can occur with ingestions as low as 3.6 mg (approximately 700 μg/kg in a toddler) 2.
Documented Cases and Mechanisms
Seizures typically manifest 24-72 hours post-ingestion, not immediately, as thyroid hormone must be absorbed and metabolized to exert its effects 2, 3.
A documented case showed a 3.5-year-old boy developed a 5-10 minute tonic-clonic seizure on day 3 after ingesting up to 3.6 mg levothyroxine, with laboratory values showing total T4 >24.0 μg/dL and TSH 0.03 mU/L 2.
Hypermotor seizures have been reported in a boy with pre-existing epilepsy who experienced 7-8 attacks per day after initiating levothyroxine for central hypothyroidism, which resolved with dose reduction 4.
One case report describes levothyroxine-induced non-convulsive status epilepticus in a patient mistakenly treated for subclinical hypothyroidism following an initial seizure episode 5.
Critical Distinction: Therapeutic vs. Toxic Doses
At therapeutic replacement doses (1.6 mcg/kg/day for adults, 25-200 mcg daily range), levothyroxine does not cause seizures 6. The medication is safe for lifelong use when properly dosed and monitored, with primary risks stemming from overtreatment rather than the medication itself 6.
When Seizures Occur in Hypothyroid Patients
Hypocalcemia, not levothyroxine itself, is a common seizure trigger in patients with thyroid disorders, particularly those with 22q11.2 deletion syndrome where hypoparathyroidism coexists 7.
Hypocalcemic seizures can occur at any age in patients with underlying parathyroid dysfunction, and these resolve with calcium supplementation rather than anticonvulsant therapy 7.
Other precipitating factors for seizures in thyroid patients include fever, ischemia, hypoxia, surgery, and certain medications including antipsychotics 7.
Management of Levothyroxine Overdose
For acute levothyroxine overdose with concern for toxicity, the FDA recommends reducing or temporarily discontinuing the dose and initiating supportive treatment 1. The National Poison Control Center (1-800-222-1222) should be contacted for current management guidance 1.
Specific Treatment Protocols
Activated charcoal (1 g/kg) should be administered if the patient presents within hours of ingestion 3, 8.
Beta-blockers (propranolol or atenolol/metoprolol) are recommended for symptomatic tachycardia (heart rate >120 bpm in children, >90 bpm in adults) 9.
Hospital monitoring for 48-72 hours post-ingestion is indicated for children with levothyroxine dose >0.1 g/kg, short interval since ingestion, symptomatic presentation, and/or free T4 >100 pmol/L 3.
Outpatient follow-up for 14 days is recommended even after hospital discharge 3.
Common Pitfalls to Avoid
Do not assume therapeutic levothyroxine causes seizures—investigate other causes including hypocalcemia, which is far more common in hypothyroid patients 7.
Do not overlook the delayed presentation of levothyroxine toxicity—symptoms may not appear until several days after ingestion 1, 2.
Do not dismiss pediatric ingestions <5 mg as benign—serious outcomes including seizures have been documented at these doses 2.
In patients with pre-existing epilepsy, recognize that levothyroxine may lower seizure threshold and require dose adjustment 4.
Evidence Quality Considerations
The evidence for levothyroxine-induced seizures comes primarily from case reports and FDA post-marketing surveillance rather than controlled trials 1, 2, 4, 3, 5, 8. However, the consistency of reports across multiple cases and the explicit FDA labeling provide strong support for this rare but real complication in overdose situations.