What clinical evidence is there to support the use of “Armour thyroid” or desiccated thyroid extract

• Desiccated thyroid extract is derived from porcine thyroid gland. One grain contains 38mcg levothyroxine (T4) and 9mcg liothyronine (T3) per 65mg of the labelled amount of thyroid. Products such as Armour® Thyroid, which contain thyroid extract, are not licensed in the UK. These products are licensed in the USA, but because they have not been approved by the US FDA as a new drug, they have not been subjected to clinical effectiveness studies.
• The amount of thyroid hormone in the thyroid gland can vary from animal to animal; the amount of T4 and T3 is measured in both the raw material and actual tablets.
• The Royal College of Physicians (RCP) recommend that levothyroxine alone is used in the treatment of hypothyroidism, underactivity of the thyroid gland. The RCP does not recommend the prescribing of additional liothyronine in any form, including Armour® Thyroid (desiccated thyroid or thyroid extract); this is inconsistent with normal physiology, has not been unequivocally proven to be of any benefit to patients, and may be harmful.
• Most liothyronine, (80%), is generated from the de-iodination of circulating T4, so a preparation of levothyroxine should provide T3 in a physiological and appropriate concentration.
• The RCP does not recommend the prescribing of desiccated thyroid because this is inconsistent with normal physiology and has not been unequivocally proven to be of any benefit to patients with hypothyroidism.
• There is a lack of good quality evidence to support the use of desiccated thyroid. Studies vary in design, size, duration and outcomes.
• There is only one small randomised, controlled, crossover trial which has compared the efficacy of desiccated thyroid with levothyroxine.