Subclinical thyrotoxicosis is defined as a low serum thyrotropin (TSH) concentration and normal serum thyroxine (T₄) and triiodothyronine (T₃) concentrations. The occurrence of these biochemical findings exemplifies the fact that very small changes in serum T₄ and T₃ concentrations can cause larger changes in serum TSH concentrations. This characteristic of pituitary-thyroid function has made measurement of serum TSH the most sensitive test of thyroid dysfunction. However, the specificity of measurements of serum TSH for identifying patients with clinically important thyroid dysfunction in whom therapy is clearly indicated is not high.

This review considers some aspects of serum TSH determinations and the relationship between subclinical thyrotoxicosis and other disorders characterized by low serum TSH concentrations and discusses the causes of subclinical thyrotoxicosis and its clinical consequences.