**2. Epidemiology**

Several population-based studies have reported that sHT is common in the general population, with a prevalence that increases with increasing age (Gharib et al., 2005a,b; Hollowell et al., 2002; Samuels, 1998; Tunbridge et al., 1977). In interpreting epidemiologic data, it should be taken into account that the ability to identify people with sHT varies by TSH assay and cut-off value, which ranged from >3 to >7 mIU/L (Kanaya et al., 2002; Rivolta et al., 1999). In this setting, it is noteworthy that some investigators suggest that the upper limit of normal for serum TSH level should be 2.5 mIU/L (Spencer et al., 1993). In support of this position is a higher prevalence of anti-thyroid antibodies in subjects with serum TSH levels >2.5 mIU/L (Vanderpump et al., 1995). Reasoning about these considerations, it is not astonishing that the reported prevalence of sHT in the general population ranges widely from 1.3% to 21%, depending on age, gender, and iodine intake (Kanaya et al., 2002; Rivolta et al., 1999; Samuels, 1998; Sawin et al., 1985; Tunbridge et al., 1977; Wang et al., 1997).

In the Wickham survey, the prevalence of sHT (TSH > 6 mIU/L) was 7.5% in women and 2.8% in men (Wickham study). An age dependent increase in serum TSH concentrations was found only when women with high serum anti-thyroid antibody values were included in the analysis; with 17.4% prevalence of sHT in women older than 75 years (Wickham study). Accordingly, in a Dutch study the prevalence of sHT in a group of middle-aged women (mean age 55 years) was 4%, the rate rising to 7.3% 10 years later (Geul et al., 1993). The higher prevalence of sHT in older people was confirmed by data from the Framingham Study, which reported a prevalence of sHT of 8.2% in men and 16.9% in women, older than 60 years and, the Colorado study (16% in men and 21% in women older than 74 yrs) (Sawin et al., 1979, Canaris et al., 2000). Overall these findings demonstrate that ageing is associated with an increased prevalence of positive anti-thyroid antibody titers and mild hypothyroidism (Mariotti et al., 1995). The prevalence of sHT varies also according to iodine intake; being higher in areas with elevated intake with respect to areas with low-normal or deficient intake (Biondi & Cooper, 2012). In this setting, the occurrence of sHT among nursing home elderly residents was 4.2% in an iodine-deficient area (urinary iodine 72 micrograms/g creatinine), 10.4% in region of obligatory iodinated salt prophylaxis (urinary iodine 100 micrograms/g creatinine) and 23.9% in an abundant iodine intake area (urinary iodine 513 micrograms/g creatinine) (Szabolcs et al., 1997).

Overall, these data show that sHT is a very frequent condition and raise the question of the opportunity for general population screening programs, although a consensus is still lacking on this topic (Biondi & Cooper, 2012). The above notwithstanding, screening older people for thyroid disorders is still suggested by some authorities, aiming at the discovery of previously undiagnosed cases of overt hypothyroidism and the monitoring of cases with subclinical dysfunction (Ladenson et al., 2000; Surks et al., 2004).
