Thyroxine Overdose Features
Thyroxine overdose manifests primarily as exogenous thyrotoxicosis with cardiovascular, neuropsychiatric, and metabolic symptoms, though the majority of cases—particularly in children—remain asymptomatic or develop only mild symptoms. 1
Clinical Presentation
The clinical features of levothyroxine overdose mirror those of hyperthyroidism but vary significantly based on dose, age, and time since ingestion:
Cardiovascular Manifestations (Most Prominent)
- Tachycardia (most common symptom, present in nearly all symptomatic cases) 2, 3
- Palpitations
- Arrhythmias, including atrial fibrillation (5.3% of cases) 2, 4
- Hypertension (particularly diastolic elevation) 2, 3
- Increased pulse and blood pressure
- In severe cases: heart failure, angina, myocardial infarction, cardiac arrest 1
Neuropsychiatric Features
- Confusion and agitation (23.2% and 20% respectively in symptomatic patients) 2, 3
- Headache
- Hyperactivity, nervousness, anxiety
- Irritability and emotional lability
- Insomnia
- Seizures (rare) 1
Metabolic and General Symptoms
- Fever and heat intolerance (18.9% of symptomatic cases) 2
- Excessive sweating
- Weight loss despite increased appetite
- Fatigue 1
Gastrointestinal Manifestations
Musculoskeletal Features
- Tremors
- Muscle weakness
- Muscle spasm 1
Other Systems
- Dyspnea
- Hair loss, flushing, rash
- Menstrual irregularities 1
Critical Timing Considerations
A crucial pitfall: symptoms may be delayed for several days after ingestion due to the time required for peripheral conversion of T4 to the active T3 form. 6, 5 This delayed presentation can be misleading and requires prolonged monitoring even in initially asymptomatic patients, preferably on telemetry until thyroid hormone levels begin declining. 5
Dose-Response Relationship
The median ingested dose in symptomatic patients is significantly higher than in asymptomatic patients (1100 μg vs 312.5 μg, p=0.003). 3 However, there is only moderate correlation between thyroid hormone elevation and clinical severity—some patients remain asymptomatic even with massive ingestions (up to 9 mg). 7, 5
Age-Related Patterns
Pediatric patients (median age 3 years for accidental ingestion):
- 80% of cases are accidental 3
- Generally asymptomatic or mild symptoms 3
- When symptomatic: tachycardia predominates
- Favorable prognosis with rare severe outcomes 2, 3
Adult patients (median age 35.5 years for intentional ingestion):
- More likely intentional/suicidal 2
- Higher risk of severe complications including coma, cardiac failure, and progression to thyroid storm 2, 6
- Significantly higher diastolic blood pressure compared to pediatric cases 2
Laboratory Findings
- Elevated free T4 and T3
- Suppressed TSH (<0.01 mU/L)
- Abnormal thyroid function tests in approximately 51% of cases 3
- Possible mild liver enzyme elevations 5
Severe Complications
Life-threatening manifestations include:
- Atrial fibrillation and other arrhythmias
- Cardiovascular collapse
- Progression to thyroid storm (rare but potentially fatal) 6
- One death reported in the literature 2
Despite the risk of toxicity with high-dose ingestions, mortality is extremely rare. 2 Over 90% of patients in large retrospective studies experienced minimal or no symptoms. 2