Introduction

The thyroid gland produces, stores, and secretes thyroxine (T4) and triiodothyronine (T3) through a negative feedback process involving the hypothalamus and pituitary gland. Thyroid dysfunction can result when any part of this process is affected, and is usually characterised by the presence of high or low levels of thyroid-stimulating hormone (TSH, secreted by the pituitary gland) and free thyroid hormones. Causes of thyroid disorders include autoimmunity (e.g., Graves' disease), infections (e.g., post-viral inflammation), other systemic medical conditions, medications (e.g., lithium, amiodarone), nutritional excesses or deficiencies (e.g., of iodine), tumours (thyroid, or rarely pituitary), trauma, or pregnancy.