What are the symptoms of a thyroxine (T4) overdose?

Symptoms of Thyroxine Overdose

Thyroxine overdose can cause serious symptoms including tachycardia, hypertension, tremors, and in severe cases, cardiovascular collapse, requiring prompt recognition and management. 1

Common Clinical Manifestations

• Cardiovascular symptoms:

  • Tachycardia and palpitations 2, 1
  • Hypertension 1, 3
  • Arrhythmias 2, 1
  • Abnormal cardiac output 2
  • Ventricular hypertrophy (with long-term overdosing) 2

• Neurological symptoms:

  • Tremors 1, 4
  • Hyperreflexia 4
  • Confusion and disorientation 1
  • Agitation and combativeness 4
  • Seizures (particularly in children) 1
  • In severe cases: cerebral embolism, coma, and death 1

• Gastrointestinal symptoms:

  • Vomiting and diarrhea 4, 5
  • Increased bowel sounds 4
  • Dehydration 5

• Other symptoms:

  • Diaphoresis (excessive sweating) 4
  • Insomnia 4
  • Weight loss (significant in prolonged cases) 4
  • Heat intolerance 6
  • Mydriasis (pupil dilation) 4

Severity and Timing of Symptoms

• Mild to moderate overdose:

  • Symptoms may be limited and manageable with beta-blockers 7, 3
  • Children often remain asymptomatic or experience only minor effects with exploratory ingestions 5

• Severe overdose:

  • Symptoms may not appear until several days after ingestion 1, 5
  • Day 2-3: Lethargy may be the initial presentation 4
  • Day 4-6: Progressive symptoms including vomiting, diaphoresis, insomnia, agitation, and confusion 4
  • Severe cases can progress to thyroid storm with profound cardiovascular effects 1, 4

Laboratory Findings

• Suppressed or undetectable TSH levels 2, 8
• Elevated free T4 and T3 levels 4, 5
• In massive overdose, free T4 levels can be extremely high (>13 mcg/dL) 4

Special Considerations

• Chronic overdosing:

  • Approximately 25% of patients on levothyroxine are inadvertently maintained on doses high enough to make TSH levels undetectable 2
  • Long-term consequences include increased risk for osteoporosis, fractures, abnormal cardiac output, and ventricular hypertrophy 2

• Pediatric cases:

  • Children may initially appear well but can develop serious symptoms days after ingestion 5
  • Even with high-dose ingestions (0.5 mg/kg), symptoms may be relatively mild but require monitoring 9

Clinical Pitfalls

• Delayed onset of symptoms:

  • Serious symptoms can occur in a delayed fashion (3-6 days after ingestion), making initial assessment misleading 4, 5
  • Patients discharged early may return with severe symptoms days later 5

• Misdiagnosis:

  • Low TSH with low free T4 may indicate central hypothyroidism rather than hyperthyroidism 6
  • Symptoms may be mistaken for other conditions like anxiety or stimulant use 4

• Monitoring duration:

  • Children with significant ingestions (>0.1 mg/kg) should be monitored for at least 48-72 hours post-ingestion and followed as outpatients for 14 days 9
  • Symptoms can persist for up to 2 weeks in severe cases 4

Remember that thyroxine overdose can be life-threatening in severe cases, and symptoms may not appear immediately. Early recognition and appropriate management are essential to prevent serious complications 1, 4.