 This study intends to examine how a common genetic pattern affects thyroid function. Recent studies have demonstrated that a substance enzyme produced by a gene has an important role in controlling circulating thyroid hormone levels. A commonly found pattern in this gene exists in many individuals and might affect the function of the enzyme. These individuals need higher doses of thyroid hormone medication in certain situations e.g. in the treatment of thyroid cancer after the thyroid gland has been removed than those individuals without the variation. 