Subclinical Hypothyroidism; Natural History, Long-Term Clinical Effects and Treatment

Subclinical hypothyroidism, defined as a mild degree of thyroid dysfunction biochemically, is a common clinical disease. It is marked by elevated serum concentrations of thyroid stim‐ ulating hormone (TSH) and normal serum concentrations of free thyroxine (fT4) and triiodo‐ thyronine (T3). Subclinical hypothyroidism can be divided into two categories, depending on the magnitude of the increase in serum TSH, with concentrations of 4.5-10 mU/L consid‐ ered mild disease and TSH >10 mU/L considered severe disease. However, the definition and clinical significance of subclinical hypothyroidism are confounded by controversies over the exact upper limit of the reference range for se um TSH.