ABSTRACT
Subclinical hyperthyroidism (SHyper) is dened as serum free thyroxine (FT4) and free triiodothyronine (FT3) concentrations at upper levels but within their respective reference ranges in the presence of low or undetectable serum thyrotropinstimulating hormone (TSH) levels.1 Subclinical hyperthyroidism is a misnomer and inappropriate term because patients with this disorder may have specic symptoms of mild thyroid hormone excess.1 In fact, thyroid hormones are sufciently increased to suppress TSH and induce adverse tissue effects.1 Highly sensitive and specic TSH assays allow experts to distinguish mild SHyper when a serum TSH level is low, but still detectable (0.1 to 0.4 mIU/L), from a more severe condition in which TSH is undetectable and fully suppressed (<0.1 mIU/L; see Table 23.1).
