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SCH is defined when serum TSH concentration is above the statistically upper limit of the reference range while serum free T4 (FT4) concentration is within its reference range (Biondi & Cooper, 2008; Surks et al., 2005). Other names for SCH include compensated, early, latent, mild, minimally symptomatic, preclinical hypothyroidism and euthyroid hyperthyrotropinemia (Chu & Crapo, 2001). It suggests a compensated early state of primary thyroid failure wherby an increased level of TSH is required to maintain notmal levels of thyroid hormones. The reference TSH levels in a normal population aged 12 and older (excluding individuals with medications or diseases that might influence thyroid function) were assessed at 0.45 to 4.12 mIU/l (2.5th–97.5th percentile) (Hollowell et al., 2002). Although there were age, gender, and ethnic group differences, they were small and it was therefore not considered necessary to adjust the reference for these parameters. Additional studies assessing the normal TSH reference in children have shown broad differences between adult and children that were dependent on the patient’s age (Elmlinger et al., 2001; Hubner et al., 2002; Kapelari et al., 2008 ; Soldin et al., 2009; Strich et al., 2012; Zurakowski et al., 1999), indicating that the definition of SCH is age-dependent. A panel of experts divided patients with SCH into two groups: patients with mildly increased serum TSH levels (4.5–10 mIU/l) and patients with more severely increased serum TSH levels (>10 mIU/l) (Surks et al., 2004).

Resistance to thyrotropin (RTSH) is a condition in which thyroid cells show reduced sensitivity to TSH. This condition is characterized by elevated serum TSH concentration, a normal or hypoplastic thyroid gland and normal to very low levels of thyroid hormones. Loss-of-function mutations in the TSH receptor gene (TSHR) lead to RTSH syndrome, presenting with either congenital hypothyroidism (CH) or subclinical hypothyroidism (Beck-Peccoz et al., 2006; Refetoff, 2003).