The adverse effects of levothyroxine in Kenya are generally associated with excessive dosage and correspond to symptoms of hyperthyroidism. They may include tachycardia, palpitations, cardiac arrhythmias, increase in blood pressure, anginal pain, headache, restlessness, excitability, insomnia, tremors, muscle weakness and cramps, heat intolerance, sweating, flushing, fever, weight loss, menstrual irregularities, diarrhoea, and vomiting. These adverse reactions usually disappear after dosage reduction or temporary withdrawal of treatment. Thyroid storm has occasionally been reported after massive or chronic intoxication and convulsions, cardiac arrhythmias, heart failure, coma, and death have occurred.

In acute overdosage, activated charcoal may be used to reduce gastrointestinal absorption if ingestion has occurred within 1 hour. Treatment is usually symptomatic and supportive; propranolol may be useful in controlling the symptoms of sympathetic overactivity. Levothyroxine overdosage requires an extended follow-up period as symptoms may be delayed for up to 6 days due to the gradual peripheral conversion of levothyroxine to tri-iodothyronine. UK licensed product information has suggested that glucocorticoids may be given to inhibit this conversion.