How should we treat patients with low serum thyrotropin concentrations?

Subclinical hyperthyroidism (SH) is defined by a low serum thyrotropin (TSH) concentration in the presence of normal levels of free thyroxine (FT4) and free triiodothyronine (FT3). However, it is helpful to distinguish between those with a detectable but low TSH value (in the 0.1-0.4 mU/l range) and those with a fully suppressed TSH (<0.1 mU/l), and we suggest the designation of grade I and grade II SH, respectively. Together, these patterns of thyroid function tests are found in 1-3% of the elderly population, and are associated with significant morbidity and mortality in longitudinal epidemiological surveys. There are a number of causes for this picture, which include endogenous thyroid disease, drug effects and concomitant nonthyroidal illness. Treatments commonly employed in the management of thyrotoxicosis are effective at correcting the biochemical abnormalities of SH but have not been shown to improve clinical outcome or symptoms. There is little good quality evidence available to guide the assessment or management of these patients. This review summarizes the clinical significance of SH and aims to provide guidance about whether we should treat patients with low serum TSH concentrations.