Food and Drug Administration (FDA) is instructing health care providers to contact patients taking unapproved thyroid medications and help transition them to FDA-approved medications. Thyroid hormone replacement medications require administering the appropriate dosage based on the patient’s thyroid hormone levels, as a dose that is too high can cause side effects, and a dose that is too low will not be as effective. Synthetic (laboratory-made) thyroid hormone replacement medications, which are FDA approved, contain only levothyroxine or liothyronine, or a combination of these two. Animal-derived thyroid medications, also called desiccated thyroid extract, have a higher risk of containing inconsistent dosages or having other impurities.