Hormonal Agent Toxicity

Thyroid Hormones
Thyroid hormones are used as replacement therapy in hypothyroidism. Levothyroxine (T4, Synthroid) is the most commonly used agent, though
levothyronine (T3) is also available.
Onset of symptoms from levothyroxine (T4) overdose may be delayed by several days as T4 requires in vivo conversion to the active hormone, T3. Hence, T3 ingestions will produce symptoms more quickly.

NOTE: Acute ingestions of levothyroxine will not produce symptoms for several days.


  • Excessive sympathetic activity


  • Tremor, confusion, agitation, hyperreflexia
  • Tachycardia, hypertension, palpitations, flushing, diaphoresis
  • Mydriasis
  • Diarrhea
  • Patient may report fever, weight loss, and heat intolerance with chronic overdose (facticious hyperthyroidism).


  • Primary hyperthyroid states (Graves disease)
  • Sympathomimetic toxicity (cocaine, amphetamines)
  • Methylxanthine toxicity (theophylline, caffeine)


  • Clinical diagnosis based on history and physical examination.
  • Elevated serum T4 and T3 levels
  • An elevated T4:T3 ratio is suggestive of chronic, excess levothyroxine intake.


  • β-Antagonists, such as propranolol or esmolol, to control the effects of excessive sympathetic activity
  • Agents that block endogenous thyroid hormone production (PTU, methimazole) have limited utility.