Subclinical hypothyroidism

Subclinical hypothyroidism (SH) is defined as a serum TSH concentration above the statistically defined upper limit of the reference range when serum free T4 (fT4) concentration is within its reference range [1, 2]. As serum TSH concentration varies over time in healthy subjects, leading to occasional abnormal values, the measurement of serum TSH and fT4 should be repeated within 3-4 months. If elevated serum TSH concentrations are confirmed and fT4 levels are within the normal range, the diagnosis of SH is made. SH is also defined as isolated hyperthyrotropinemia, compensated hypothyroidism, preclinical hypothyroidism, mild thyroid failure, or mild hypothyroidism. The diagnosis of SH is mainly based on a biochemical evaluation because it has been observed that most patients exhibit few or no signs or symptoms of thyroid dysfunction. Many studies suggest that some patients do indeed have clinical or functional manifestations of mild thyroid failure that are more frequent than in age-matched controls [2].