What are the symptoms to look out for in a levothyroxine (Synthroid) overdose?

Levothyroxine (Synthroid) Overdose: Clinical Monitoring Guide

Immediate Symptoms to Monitor (Within Hours to Days)

The most critical symptoms to watch for after levothyroxine ingestion include signs of hyperthyroidism, though importantly, symptoms may not appear until several days after ingestion. 1

Early Manifestations (Hours to 2-3 Days)

• Gastrointestinal symptoms including vomiting, nausea, and diarrhea may occur within the first 9-24 hours 1, 2
• Neurological changes such as confusion, disorientation, hyperactivity, or paradoxically lethargy can develop early 1, 2
• Pupillary abnormalities including hippus (rhythmic pupil dilation/constriction) or abnormal pupillary light reflexes may be observed 2
• Cardiovascular effects including tachycardia, tachypnea, and palpitations typically emerge within 24-48 hours 2

Delayed Manifestations (3-23 Days Post-Ingestion)

The FDA label explicitly warns that symptoms may not necessarily be evident or may not appear until several days after ingestion, making prolonged observation essential 1. Key delayed symptoms include:

• Severe hyperthyroid symptoms such as tremor, heat intolerance, anxiety, and weight loss typically manifest 6-23 days post-ingestion 3
• Cardiac complications including atrial fibrillation, abnormal cardiac output, and ventricular hypertrophy can develop with sustained thyroid hormone excess 4, 5
• Hepatotoxicity with elevated liver enzymes (ALT may peak around day 6 post-ingestion) 2

Life-Threatening Complications

Monitor closely for severe complications that have been reported with levothyroxine overdose, including cerebral embolism, shock, coma, seizures, and death. 1

• Seizures can occur, particularly in children (documented in a 3-year-old who ingested 3.6 mg) 1
• Cardiovascular collapse with shock and severe hypotension represents a medical emergency 1
• Altered mental status progressing to coma requires immediate intensive care 1
• Cerebral embolism is a rare but documented complication 1

Critical Monitoring Parameters

Cardiovascular Monitoring

• Continuous cardiac monitoring for tachycardia, arrhythmias (especially atrial fibrillation), and blood pressure abnormalities 4, 5
• Watch for paradoxical effects including both hypertension and hypotension depending on timing and dose 2

Neurological Assessment

• Serial mental status examinations to detect confusion, disorientation, agitation, or declining consciousness 1
• Monitor for seizure activity, particularly in pediatric patients 1
• Assess for tremor, hyperreflexia, and movement disorders 2

Laboratory Monitoring

• Serum thyroid hormone levels (T4 and T3) should be measured, though T4 may remain elevated for up to 36 days post-ingestion 2
• Liver function tests (ALT/AST) as hepatotoxicity can develop, typically peaking around day 6 2
• Electrolytes and renal function to monitor for secondary complications 6

Timeline Considerations

The critical observation period extends well beyond initial presentation:

• First 24 hours: Watch for GI symptoms, early neurological changes, and initial cardiovascular effects 1, 2
• Days 2-7: Peak risk for severe hyperthyroid symptoms, cardiac arrhythmias, and hepatotoxicity 3, 2
• Days 7-23: Continued risk for delayed manifestations of thyrotoxicosis 3
• Up to 36 days: Serum T4 levels may remain elevated, requiring extended monitoring in severe cases 2

Special Population Considerations

Elderly Patients and Those with Cardiac Disease

• Higher risk for cardiac decompensation, angina, or arrhythmias even with therapeutic doses, making overdose particularly dangerous 4, 7
• Lower threshold for ICU admission and more aggressive cardiac monitoring 4

Pediatric Patients

• Increased seizure risk documented in children 1
• More rapid onset of symptoms may occur compared to adults 2

Common Pitfalls to Avoid

• Do not assume safety based on initial asymptomatic presentation - symptoms may be delayed up to several days 1
• Do not discontinue monitoring prematurely - thyroid hormone levels can remain elevated for weeks 2
• Do not overlook subtle early signs such as pupillary changes or mild tachycardia that may herald more severe toxicity 2
• Do not miss concurrent adrenal insufficiency - if present, starting or increasing thyroid hormone can precipitate adrenal crisis 3, 4

When to Escalate Care

Immediate ICU transfer is warranted for:

• Altered mental status, confusion, or declining consciousness 1
• Seizure activity 1
• Hemodynamic instability (shock, severe hypertension, or hypotension) 1
• Cardiac arrhythmias, particularly atrial fibrillation 4, 5
• Signs of cerebral embolism or stroke 1