Iodine is an essential trace element, vital for healthy thyroid function. Adequate levels are required to enable the production of T3 and T4 thyroid hormones, whilst also being required in other areas of health. Deficiencies can lead to impaired heat and energy production, mental function and slow metabolism. Urine iodine is one of the best measures of iodine status. This test is not performed as a loading test, but can be used to establish existing levels or to monitor iodine supplementation.