Search

Subclinical hypothyroidism (sHT) is defined as serum thyrotropin-stimulating hormone (TSH) concentration above the statistically defined upper limit of the reference range in the face of serum free T4 (FT4) and free T3 (FT3) concentrations within the normal range (Ross, 2004). sHT is a frequent condition among the general population, especially middle-aged and elderly women (Canaris et al., 2000). Depending on the degree of TSH elevation, sHT has been associated with hyperlipidemia, intermediary metabolism alterations, arterial hypertension and cardiovascular disease (CVD) as well as neuropsychiatric features, including cognitive impairment (Ashizawa et al., 2010; Biondi & Cooper., 2008; Cappola et al., 2006; Ceresini et al., 2010; Mitrou et al., 2011; Monzani et al., 2006; Rodondi et al., 2010; Tan et al., 2008; Tappy, 1987). Interestingly, the analysis of variation in thyroid function tests in healthy volunteers has shown that the physiological individual reference ranges for test results are narrow compared with laboratory references (Andersen et al., 2002). This finding suggests that a test result within laboratory reference limits is not necessarily normal for an individual. Giving that serum TSH responds with logarithmically amplified degree to minor changes in serum T4 and T3, abnormal serum TSH may indicate that T4 and T3 are not normal for an individual although still within the laboratory references. These data point out that the distinction between subclinical and overt thyroid failure (elevated serum TSH and low T4 and/or T3) is somewhat arbitrary. Indeed, for the same degree of thyroid function abnormality, the diagnosis depends to a considerable extent on the position of the patient’s normal set point for T4 and T3 within the laboratory reference range.