The normal TSH reference range: what has changed in the last decade?

Serum TSH assessment is the most sensitive screening test for the diagnosis of thyroid dysfunction in the absence of pituitary or hypothalamic disease. This test has been used increasingly in the last decade to detect subclinical thyroid dysfunction (STD). Recent data suggest that STD is a common disorder that may be associated with important adverse events (1–5). The American Association of Clinical Endocrinologists (AACE) and American Thyroid Association (ATA) 2012 guidelines recommend treatment of subjects with persistent increased serum TSH levels10 mIU/L and undetectable serumTSH(0.1 mIU/L) (6, 7). The treatment of mild thyroid hormone excess (TSH 0.1–0.4 mIU/L) or deficiency (serumTSH10mIU/L) is controversial (1, 2). However, what is the normal TSH reference range? Obviously, this issue is critical in deciding whether or not to treat patients with mild STD and in identifying individuals with high-normal or low-normal serum TSH. It is also important in defining the TSH target level in patients receiving thyroid hormone replacement therapy.