Thyroid hormone is available in the synthetic forms liothyronine (triiodothyronine, or T3), levothyroxine (tetraiodothyronine, or T4), and liotrix (both T3 and T4) and as natural desiccated animal thyroid (containing both T3 and T4). Dosage equivalents are listed in Table II–57. Despite concern about the potentially life-threatening manifestations of thyrotoxicosis, serious toxicity rarely occurs after acute thyroid hormone ingestion.