Therapeutic options are related to the time elapsed after the ingestion of a large number of tablets of L-thyroxine and the actual beginning of emergency therapy ( Table 3). Acute levothyroxine overdose is much more common in children compared to adolescents and adults. Therapeutic recommendations are made based only in the review of the available literature concerning a relatively large number of patients, most of them children. As already mentioned serious complications are not common, but they can appear days after ingestion, and therefore the patients should be closely monitored. One-time ingestion of up to 3 mg thyroxine rarely causes symptoms in adult or children. Medical consensus has indicated that serious symptoms are less frequent in children even though children usually have higher mean plasma levels of T4 and T3 than adults for the same overdose of LT4 ingested. Interestingly the onset of symptoms and signs ( Table 1) may be delayed for up to 3 to 10 days and does not correlate closely with plasma levels of serum total T4 and total T3. However, only one fatality has been reported. Cardiac effects aside from tachycardia are seldom seen in young adults but may occur in middle age and older adults, with reported arrhythmias and acute myocardial infarction. Rarely more serious late effects occur, including coma, convulsions, and acute psychosis.
At this time, the common clinical signs and symptoms include nervousness, insomnia, mild tremor of hands, tachycardia, mild elevation of body temperature, blood pressure elevation, and loose stools. Rarely, the overdose is discovered immediately and the patient is brought to the hospital 6-12 hours after the ingestion. Patients in this age range may ingest a full flask of LT4 with 90-100 tablets (100 or 150 mcg/tablet). In children and adolescents, the clinical course is often very mild. Agents that inhibit T4>T3 conversion provide one approach to treatment. Both thyroxine and triiodothyronine levels in serum rise within 1-2 hours of ingestion.
Ingested thyroxine, which is itself probably of modest physiologic significance, is rapidly partially converted to triiodothyronine (T3), the active form of thyroid hormone. Thyroid hormone pills used to treat hypothyroid dogs typically contains a much higher dose of thyroid hormone and if mistakenly taken by humans can lead to thyroxine poisoning. In some reports thyroxine preparations by a pharmacist had an erroneous LT4 dosage. In some localities thyroxine may be obtained at drugstores without prescription (mostly in the generic form). It may occur intentionally in young and older adults in an attempt to lose weight, with suicidal intentions, or for undeclared purposes.
A massive L-Thyroxine (T4) overdose may be accidentally and unintentionally ingested, most commonly by children and adolescents.
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