Toxicology – Thyroid Hormone Toxicity

Source

Thyroid hormones such as T3 (triiodothyronine) and T4 (levothyroxine) are prescribed for hypothyroidism. Rarely, thyrotoxicosis can also occur after accidental ingestion of animal thyroid tissue in contaminated meat.

Typical Presentation

A patient may initially appear well after taking excess thyroid hormone, especially T4, and then develop symptoms hours to days later. Common complaints include palpitations, anxiety, nausea, vomiting, headache, and a rapid heart rate.

Clinical Features

Toxicity produces a hyperadrenergic picture resembling a sympathomimetic toxidrome. Possible findings include:

• Tachycardia and palpitations
• Hypertension
• Hyperthermia and sweating
• Rapid breathing
• Nausea, vomiting, and diarrhea
• Tremor, restlessness, and insomnia
• Headache and confusion
• Seizures in severe cases

ECG findings may include sinus tachycardia or supraventricular tachyarrhythmias.

T3 toxicity usually develops more rapidly, while T4 toxicity can be delayed because T4 must first be converted to active T3.

Mechanism of Action

Thyroid hormones increase basal metabolic activity and enhance sensitivity to catecholamines. T3 is the main biologically active form, while T4 acts largely as a precursor that is converted to T3 in peripheral tissues.

Management

Treatment is mainly supportive:

• Cardiac and temperature monitoring
• Beta-blockers, particularly propranolol, may help control tachycardia, tremor, palpitations, and anxiety
• Benzodiazepines may be used for severe agitation or seizures
• In significant T4 toxicity, medications that reduce peripheral conversion of T4 to T3 may be considered under specialist guidance

Because symptoms can be delayed, continued observation and appropriate follow-up may be necessary.

Key Points

• T4 overdose may have a delayed presentation.
• Thyroid hormone toxicity commonly resembles a sympathomimetic state.
• T3 acts faster because it is already biologically active.
• Most uncomplicated acute exposures improve with supportive treatment.